8.14 cherry-picks (#12418)

8.14 cherry-picks (#12418)

* Improve thread safety (#12395)

See: https://vaadin.com/forum/thread/17522264/concurrentmodificationexception-in-vaadin-shared-on-karaf-4-2-x

* Fix incompatible selenium version in test module. (#12397)

* Fixed a dependency version in a karaf test module. (#12399)

* Checkstyle tweaks to DateField widgets. (#12400)

- Added and updated JavaDocs.
- Updated comments.
- Updated to use non-deprecated method calls.
- Removed unnecessary warning suppressions.
- Suppressed warnings for unavoidable deprecation.

* fix: set Vaadin session attribute using lock in reinitializeSession (#12401)

* Cherry picked unit test from Flow

See https://github.com/vaadin/flow/pull/11538

* Fix missing import

* Cherry pick fix from Flow

* deprecate vaadin-snasphots repo (#12405)

* deprecate vaadin-snasphots repo

* Update chrome version to 93

* add more screenshots

* fix: Add MPR UI id request parameter (#12412)

* fix: Add MPR UI id request parameter

Related-to https://github.com/vaadin/multiplatform-runtime/issues/85

* test: Remove redundant non-empty param test

* test: Remove leftovers

* fix: Init window.mprUiId earlier than window.vaadin

* Add missing '='

* Update links shown by license checker (#12402)

vaadin.com/pro does no longer have the info

* fix: Add row limit to DataCommunicator row data requests (#12415)

* Add row limit to DataCommunicator row data requests

* Add missing constant

* Add unit test

* Add test for extending Grid

* Fixed test

Co-authored-by: Tatu Lund <tatu@vaadin.com>
Co-authored-by: Anna Koskinen <Ansku@users.noreply.github.com>
Co-authored-by: Zhe Sun <31067185+ZheSun88@users.noreply.github.com>
Co-authored-by: Mikhail Shabarov <61410877+mshabarov@users.noreply.github.com>