Make searching issues by keyword case insensitive on DB (#14848)

Make searching issues by keyword case insensitive on DB (#14848)

Most DBs apart from SQLite will use a default Collation that is not case insensitive.
This means that SearchIssuesByKeyword becomes case sensitive for db indexing - in
contrast to the bleve and elastic indexers.

This PR simply uses UPPER(...) to do the LIKE - and although it may be more efficient
to change collations this would be a non-trivial task.

Fix #13663

Signed-off-by: Andrew Thornton <art27@cantab.net>