General: Fail on load if using unsupported WP.

[kraftbj]

Replaces and closes #11572

Every normal way of updating Jetpack will not allow us to update to on an unsupported version of WordPress, but we can just be extra careful in the event someone runs an unexpected upgrade (e.g. via FTP, Git, etc).

Changes proposed in this Pull Request:

• Abort Jetpack if running on an unsupported WP version.

Testing instructions:

• JN site running WP 4.9
• Install, activate, and setup Jetpack 7.1.1
• Using git, checkout this branch (unbuilt is fine for testing).

If checking out master, it'll hard fatal. If checking out this branch, Jetpack will still be active, but there will be an admin_notice requesting an upgrade to keep using Jetpack.

Proposed changelog entry for your changes:

• Harden forced support for supported WordPress versions.

[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 2, 2019.
Scheduled code freeze: March 26, 2019

Generated by 🚫 dangerJS against 3d4e5e9

[jeherve]

Quick i18n change.

[mdawaffe]

Works well.

I'm not sure if this is in scope or not, but going to /wp-admin/admin.php?page=jetpack in WordPress 4.9 just displays the "Sorry, you are not allowed to access this page." error (since the jetpack menu item is never created).

Should we add a stub menu item there that just contains the new admin notice?

[enejb]

Would it make sense to provide the user with a link to support to help them achive updating of WordPress.
also does do we show this to everyone.
Even non admins?

[kraftbj]

As it stands, it shows to everyone. I'm okay not changing behavior though because it would only been shown on sites that upgraded to Jetpack in some atypical way and with Jetpack not working, it would be pretty apparent to anyone who used any feature.

[jeherve]

This works well in my tests. 👍 It's not as pretty as the other banners you can get when Jetpack actually runs so it may not be as visible (it's easier to miss if you have other banners already installed), but it does the trick!

[jeherve]

cc @danjjohnson This is important, as it will potentially happen to some of our users in the next release. I would recommend preparing a predef explaining to folks the need to update to WP 5.0, the mininum supported version of WordPress, if they want to continue to use Jetpack. We may mention the classic editor plugin in there since I assume some people have refrained from updating because their site is not ready for the block editor.

[kraftbj]

To add on, this is normal for Jetpack (only supporting the current and last previous major version of WP), so we're only being a bit extra cautious since it's been awhile since we bumped previous versions due to the long development cycle of 5.0.