Add support for 'Tested up to' header in the main plugin file

[SergeyBiryukov]

Background:

• #2952 added support for Requires PHP plugin header in readme.txt.
• #4514 added support for Requires at least and Requires PHP headers in the main plugin file.

Since [9050], the preferred placement for Requires at least and Requires PHP headers is the plugin's main file, rather than readme.txt.

(Per discussion with @Otto42 in ​#WP46938, WordPress core should read the headers it needs from the plugin's PHP file, while the readme.txt is intended for the WordPress.org directory.)

It occurred to me that the Tested up to header still can only be defined in readme.txt. Even though core doesn't read it from the file at the moment, I think it makes sense to be able to define all three headers (Requires at least, Tested up to, Requires PHP) in the same place, for consistency.

Otherwise, developers who define them in the plugin's main file could be confused as to why two of them work as expected, but the third one does not.

[SergeyBiryukov]

Replying to SergeyBiryukov:

Otherwise, developers who define them in the plugin's main file could be confused as to why two of them work as expected, but the third one does not.

See an example in ​#WP55694, where the Requires at least and Requires PHP headers work as expected in the main plugin file, but Tested up to does not.

[ipajen]

"Requires at least" is kind of helpful as minimum requirement Knowlege but with a “Tested up to PHP version” would let us know if a plugin is working with Major new PHP versions like 8.x

​https://wordpress.org/support/topic/tested-up-to-php-version-for-plugins/?view=all#post-16121147

[dd32]

In 13637:

Plugin Directory: Support the Tested Up To header in the main plugin file.

This is as the other similar metadata fields are preferred from the plugin file, so it makes sense to have all versions co-located.

Fixes #4621.