DOC-2174: added documentation for Mentions plugin's getUsers API deprecation to 6.0-release-notes-premium-changes.adoc and migration-from-5x.adoc

[FarzadHayat]

Ticket: DOC-2174, getUsers API deprecation in v6 not documented.

Changes:

• added getusers-api-alternative demo to 6.0 release notes and the 5 to 6 migration guide

Pre-checks:

• Branch prefixed with feature/6/ or hotfix/6/
• Changelog entry added
• Files has been included where required (if applicable)

Review:

• Documentation Team Lead has reviewed

[FarzadHayat]

Just thought I would put up a first draft, open to suggestions!
Feel free to provide your thoughts if you think the changes could be structured better.

Some questions regarding the demo for reviewers:

• are you happy with the inclusion of the interactive example inside 6.0-release-notes-premium-changes.adoc?
• currently, if a viewer switches away from the demo to the HTML or JS tab, the editor will restart and lose the current list of users which is confusing. what are your thoughts about modifying the custom getUsers function to include all mentions in the document rather than just ones in the current editor session?

[kemister85]

@FarzadHayat Well done, i think this is a good improvement as discussed.