[DOCS] Document deprecation of storing nanosecond resolution on date field

[pgomulka]

relates #78921

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?
• If submitting code, have you built your formula locally prior to submission with gradle check?
• If submitting code, is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed.
• If submitting code, have you checked that your submission is for an OS and architecture that we support?
• If you are submitting this code for a class then read our policy for that.

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[pgomulka]

@jrodewig this is meant to be a deprecation in both 7.16 and 8.0. Is there a separate section for deprecations only?
I placed this in a breaking changes for now.

[jrodewig]

Thanks @pgomulka. I left some suggestions with edits.

@jrodewig this is meant to be a deprecation in both 7.16 and 8.0. Is there a separate section for deprecations only?

If we start deprecation in 7.16, we only need to add deprecation docs to the 7.16 migration guide: https://github.com/elastic/elasticsearch/edit/7.x/docs/reference/migration/migrate_7_16.asciidoc

We don't need to add these changes to the 8.0 migration guide at all. Instead, move these changes to the above file and rebase this PR against 7.x.

[pgomulka]

in that case I will close this PR in favour of the 7.x PR
#79554