PHP 8.0: fix fatal error "Non-static method cannot be called statically"

[jrf]

PHP 8 elevates the error level of a number of errors, including the Non-static method cannot be called statically error.
This would previously throw a deprecation warning, but will now throw a fatal error.

In the Text_Diff_Engine_shell::diff() method, the Text_Diff::_getTempDir() method is called statically, which will cause a fatal error in PHP 8.

See: ​https://3v4l.org/jRJK5

While this is an external library, it is no longer maintained externally, so fixing this in Core by explicitly declaring the Text_Diff::_getTempDir() method as static.

The method doesn't use $this, so changing it to static is safe and is in line with how the method was documented.

Trac ticket: https://core.trac.wordpress.org/ticket/51559

[SergeyBiryukov]

In 49185:

Revisions: Explicitly declare the Text_Diff::_getTempDir() method as static.

This fixes a "Non-static method cannot be called statically" fatal error on PHP 8.

Props jrf.
Fixes #51559.

Closing as committed.