Prepare 2.9.0 release

[eclarke1]

This issue is to track preparation of the upcoming 2.9.0 release up until publishing, which is due February 20, 2024.

• Create release/2.9.0 branch closer to the release date
• Finalize scope and punt unfinished pull requests to following release
• Prepare the release (Tuesday February 20, 2024)

[eclarke1]

@joemcgill @westonruter did we manage to create this branch so we can commence testing today? Also updating the date here to be Tuesday Feb 20

[joemcgill]

The branch is ready, but I'm reviewing why unit tests are currently failing.

[joemcgill]

Looks like the Unit Test failures showing in the branch are inaccurate and are pointing to the wrong commit in our repo. Regardless, I set up a way to manually trigger these in #990 and am porting them to the 2.9.0 branch (#991), which will also re-trigger the test workflow.

[joemcgill]

Prepped the PR for the release ☝🏻, which we won't merge until the release party.

[westonruter]

Testing instructions for what's new in this release:

1. Make sure that Auto Sizes and Speculation Rules plugins are listed on the settings screen and that the plugins can be installed (PR).
2. Install the plugin on a read-only filesystem (see disable-file-mods.zip mini plugin) and make sure that the messages for installing the standalone plugins make sense (issue).
3. When standalone plugins have been installed, make sure they appear in the PL generator tag (PR).
4. On Tools > Server-Timing, enable output buffering and add render_block_core/site-logo to the list of filters that are timed, and make sure that this shows up in the Server-Timing when accessing the page (issue).

[westonruter]

My testing:

1. Check out update/2.9.0-release-prep
2. Run git archive --output performance-lab.zip HEAD
3. Spin up a new WP site at https://wp.new/
4. Upload the performance-lab.zip to the site and activate

Now for testing what is new:

For points 1 & 2, I can see that the new plugins are listed and when file mods are disabled, the installation buttons and messages are modified:

File Mods Enabled	File Mods Disabled

Note the plugin assets for Auto Sizes and Speculation Rules will be updated before.

For the generator tag, I activated all of the plugins and I see them listed as expected:

<meta name="generator" content="Performance Lab 2.9.0; modules: images/webp-support; plugins: webp-uploads, dominant-color-images, performant-translations, auto-sizes, speculation-rules">

In regards to Server-Timing, I am able to see the render_block_core/site-logo timing now show up: