Make prominent the suggestion to migrate to NoteDb in v2.16.x
Make prominent the suggestion to migrate to NoteDb in v2.16.x

The existing recommendation to wait until v2.16.x for the NoteDb
migration was not prominent enough: make it specific to the important
notes and remove the statement that NoteDb is "officially supported",
because of the existing unresolved P0 bug during the v2.15 to v2.16
NoteDb migration.

Bug: Issue 10239
Change-Id: If09068942b4c9b96835f255a56bf3dc87152ed79

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