[Enterprise Search] Remove beta status/notification removal

[cee-chen]

Summary

Removes beta status from our plugin by removing the sidebar notification and updating our README.

Before

After

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

Release note

The Enterprise Search Kibana interfaces are now out of beta status.

[cee-chen]

NOTE: This is a draft PR and is mostly meant to be a reminder to remove the various beta code/copy in 7.16. If there aren't any major merge conflicts, ideally we'd be able to pull this PR out of draft and merge upstream and call it a day.

closes https://github.com/elastic/app-search-team/issues/1986

[kibanamachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: 83bd5ff
• Storybooks Preview
• Pipeline Steps (look for red circles / failed steps)
• Interpreting CI Failures

Failed CI Steps

• Check Plugin List Docs

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
enterpriseSearch	1468	1458	-10

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
enterpriseSearch	2.1MB	2.1MB	-7.8KB

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[byronhulcher]

lgtm for 7.16 🚀