Changelog: final changes for the 9.6 release

[jeherve]

Changes proposed in this Pull Request:

• This PR brings in the changes that we made to the changelog for the 9.6 release, from the PRs that were cherry-picked to the release branch.

Jetpack product discussion

• N/A

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

• Check for typos.

[github-actions]

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Jetpack plugin:

• Next scheduled release: May 4, 2021.
• Scheduled code freeze: April 26, 2021.

[anomiex]

Should we leave the "Other" section out of 9.6, since we're really only starting to populate it in 9.7?

Suggested change

	### Other
	- Admin Page: Do not show an error message with JSONP is disabled. The admin page does not use it.
	- Readme: update link to changelog file. No changelog entry necessary.

[jeherve]

I'm fine with leaving it there, to be honest, since that's not very visible.