Remove warnings about pre-release content in documentation (#9737)

Resolves #9191

documentation/advanced/advanced-dragndrop.asciidoc

@@ -9,8 +9,6 @@ layout: page
 
 ((("Drag and Drop", id="term.advanced.dragndrop", range="startofrange")))
 
-IMPORTANT: This feature is currently being developed and only available in the Framework 8.1 prerelease versions, starting from 8.1.0.alpha1.
-
 Dragging an object from one location to another by grabbing it with mouse,
 holding the mouse button pressed, and then releasing the button to "drop" it to
 the other location is a common way to move, copy, or associate objects. For

documentation/advanced/advanced-osgi.asciidoc

@@ -7,8 +7,6 @@ layout: page
 [[advanced.osgi]]
 = Vaadin OSGi Support
 
-IMPORTANT: The OSGi support as described here is currently being developed and only available in the Framework 8.1 prerelease versions, starting from 8.1.0.beta1.
-
 Vaadin applications can be deployed on an OSGi compatible servlet container, or on Liferay 7 as OSGi portlets.
 
 An OSGi application typically consists of multiple bundles that can be deployed separately. Multiple versions of each bundle can be present, and the OSGi runtime resolves their dependencies at run-time based on bundle manifests.

documentation/components/components-customcomponent.asciidoc

@@ -22,8 +22,6 @@ The benefit of wrapping a layout composite is mainly encapsulation - hiding the
 implementation details of the composition. Otherwise, a user of the composite
 could rely on implementation details, which would create an unwanted dependency.
 
-IMPORTANT: The [classname]#Composite# component is currently being developed and only available in the Framework 8.1 prerelease versions, starting from 8.1.0.alpha6.
-
 To create a composite, you need to inherit [classname]#Composite# or
 [classname]#CustomComponent# and set the __composition root__ component in the
 constructor. The composition root is typically a layout component that contains

documentation/components/components-grid.asciidoc

@@ -528,8 +528,6 @@ Column<Integer> birthYear = grid.addColumn(Person::getBirthYear,
 [classname]#ProgressBarRenderer#:: Renders a progress bar in a column with a [classname]#Double# type. The value
 must be between 0.0 and 1.0.
 
-IMPORTANT: The following renderers are to be released in version 8.1 and are currently only available in link:https://vaadin.com/releases[pre-release versions] of the framework, starting from 8.1.0.alpha3.
-
 [classname]#LocalDateRenderer#::
 Formats a column with the [classname]#LocalDate# type.
 The renderer can be constructed with a [classname]#DateTimeFormatter#, or with a custom pattern string.

documentation/components/components-tree.asciidoc

@@ -12,8 +12,6 @@ ifdef::web[]
 image:{live-demo-image}[alt="Live Demo", link="http://demo.vaadin.com/sampler/#ui/grids-and-trees/tree"]
 endif::web[]
 
-IMPORTANT: The [classname]#Tree# component is currently being developed and only available in the Framework 8.1 prerelease versions, starting from 8.1.0.beta1.
-
 [[components.tree.overview]]
 == Overview
 

documentation/components/components-treegrid.asciidoc

@@ -12,8 +12,6 @@ ifdef::web[]
 image:{live-demo-image}[alt="Live Demo", link="http://demo.vaadin.com/sampler/#ui/grids-and-trees/treegrid"]
 endif::web[]
 
-IMPORTANT: The [classname]#TreeGrid# component is currently being developed and only available in the Framework 8.1 prerelease versions, starting from 8.1.0.alpha1.
-
 [[components.treegrid.overview]]
 == Overview
 

documentation/datamodel/datamodel-hierarchical.asciidoc

@@ -7,8 +7,6 @@ layout: page
 [[datamodel.hierarchical]]
 = Hierarchical Data
 
-IMPORTANT: The [interfacename]#HierarchicalDataProvider# is currently being developed and only available in the Framework 8.1 prerelease versions, starting from 8.1.0.alpha1.
-
 The [classname]#Tree# and the [classname]#TreeGrid# components allow you to show data with hierarchical relationships between items.
 That data can be populated by on-demand from a back end by implementing the [interfacename]#HierarchicalDataProvider# interface. If you have the data available in-memory on the server,
 you use the collection style API of [classname]#TreeData# and then pass it to a [classname]#TreeDataProvider#. This chapter introduces the hierarchical data providers and how they work.

documentation/portal/portal-osgi.asciidoc

@@ -7,8 +7,6 @@ layout: page
 [[portal.osgi]]
 = OSGi Portlets on Liferay 7
 
-IMPORTANT: The OSGi support as described above is currently being developed and only available in the Framework 8.1 prerelease versions, starting from 8.1.0.beta1.
-
 Lifeary 7 supports modular portlet development using OSGi, and enables e.g.
 using multiple different Vaadin versions in different portlets on a page.