Twenty Twenty-One: Remove use of `filemtime()`

[desrosj]

In Twenty Twenty-One, there are two instances of filemtime() to create a version number for a CSS or JS asset.

This should be removed in favor of wp_get_theme()->get( 'Version' ).

[joyously]

I think that was supposed to be temporary...

[SergeyBiryukov]

Since there will be a new version of the theme released with WordPress 5.8, would it make sense to include this too, rather than wait for 5.8.1?

[desrosj]

No objection here! Only put it into 5.8.1 to try and allow a newer contributor some time to grab it!

I just shared the ticket in #core on Slack. Let's see if anyone is able to create a patch!

[ryelle]

Out of curiosity, is there a reason to not use filemtime, or just that it's stable now and should use wp_get_theme()->get( 'Version' )?

Trac ticket: https://core.trac.wordpress.org/ticket/53502

[peterwilsoncc]

I've set up a pull request for this change. ​https://github.com/WordPress/wordpress-develop/pull/1432

[peterwilsoncc]

Replying to ryelle:

Out of curiosity, is there a reason to not use filemtime, or just that it's stable now and should use wp_get_theme()->get( 'Version' )?

In certain set ups or using some deployment techniques the file's modified time can change more frequently than the file is modified so browser and proxy caches end up getting broken more often than needed.

[aristath]

Makes sense to change it to use the theme-version.

[desrosj]

In 51236:

Twenty Twenty-One: Use the theme version when enqueueing theme assets.

This avoids having to interact with the filesystem and ensures browser and proxy caches are only cleared when the file is actually updated.

Props peterwilsoncc, ryelle, aristath, SergeyBiryukov.
Fixes #53502.