General: update WordPress version requirements

[jeherve]

Changes proposed in this Pull Request:

• With the upcoming release of WordPress 5.9, our plugins will now require the previous version of WordPress, version 5.8.
• Let's also update to reflect that we've tested our plugins with the upcoming version of WordPress.

Jetpack product discussion

• WordPress 5.9 compatibility #21322

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

• No

Testing instructions:

• Is CI happy?

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello jeherve! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer and confirm D70236-code works as expected before merging this PR. Once this PR is merged, please commit the changes to WP.com. Thank you!
This revision will be updated with each commit to this PR

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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Jetpack plugin:

• Next scheduled release: January 11, 2022.
• Scheduled code freeze: January 3, 2022.

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Backup plugin:

• Next scheduled release: January 4, 2022.
• Scheduled code freeze: December 27, 2021.

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Boost plugin:

• Next scheduled release: January 31, 2022.
• Scheduled code freeze: January 24, 2022.

[kraftbj]

5.9 will be delayed. https://make.wordpress.org/core/2021/11/17/wordpress-5-9-beta-1-delayed/

Let's see where they land with schedule and then decide if we merge this for 10.4 or the week-later-than-usual 10.5 January release.

[sdixon194]

CI still happy

[github-actions]

Great news! One last step: head over to your WordPress.com diff, D70236-code, and commit it.
Once you've done so, come back to this PR and add a comment with your changeset ID.

Thank you!

[jeherve]

r236660-wpcom