General: update minimum WordPress version for Jetpack

[jeherve]

Changes proposed in this Pull Request:

WordPress 5.4 is scheduled for release before the next version of Jetpack will ship.

Let's consequently aim to update Jetpack to keep our WP-1 support standard.

I've taken the opportunity to try again to switch on Unit Tests on PHP 7.4 (see #14171) (Edit: nope, that still does not work, had to revert), and remove old shims.

Related PRs:

• Related Posts: Adding support for up to 6 related posts #11220
• Crowdsignal: Shortcodes: Clean up Crowdsignal oEmbed regex #11540
• Time utils: WP 5.3: Use modern wp_timezone #13521

Testing instructions:

• See PRs above for each functionality change.

Proposed changelog entry for your changes:

• General: Jetpack now supports WordPress 5.3 and newer.

[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, review, and approve D40014-code before merging this PR. Thank you!

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: April 7, 2020.
Scheduled code freeze: March 31, 2020

Generated by 🚫 dangerJS against 0a3771e

[kraftbj]

The previous branch may fail in Travis until 5.4 actually ships.

[kraftbj]

Travis may need to be 7.4snapshot instead of 7.4 to be added to the matrix.

[jeherve]

r204379-wpcom