Refactor: Deprecate msort

[yookoala]

Description

• Rewrite all usages of msort in core to usort with documentation.
• Properly document msort with phpdoc and mark it deprecated.

Motivation and Context

• Part of the effort to retire old functions. See Meta: retire old functions in functions.php #1689.
• msort is a really old function in Gibbon (First introduced to git repo in 2286c9b, which was when Gibbon first migrated to git). I'm not sure if it is used elsewhere in modules (although I've searched all modules to found no reference to it). Might be safer to make it deprecated here.

How Has This Been Tested?

• Locally.
• CI.

[yookoala]

Test failed because arrow function used in this is a PHP 7.4 feature, but the composer PHP version config is now at 7.3. phpstan installed does not support PHP 7.4 syntax, hence the strange error message. #1753 will fix this issue.

[SKuipers]

Thanks, can you rerun the checks now that #1753 is merged? It usually gives me the option to, but I'm not seeing it this time.

[yookoala]

@SKuipers: All tests are past now :-)

[SKuipers]

Thanks! 👍