PHP 8.1 compatibility: replace FILTER_SANITIZE_STRING

[sjinks]

PHP 8.1 deprecated FILTER_SANITIZE_STRING. This PR replaces it with other alternatives.

Ref: Automattic/vip-go-mu-plugins#2667

[jeffpaul]

Per yesterday's bug scrub, @georgestephanis feels the sanitize string is reasonable and could be good to merge, but is not familiar with filter_callback and not positive if core code style still recommends using array( ) over [ ] shorthand.

[GaryJones]

not familiar with filter_callback

FILTER_CALLBACK is documented here, but essentially is a way to ignore the built-in FILTER_* constants, and set a callback function instead.

not positive if core code style still recommends using array( ) over [ ] shorthand

WPCS still requires longhand array syntax, so that's the code suggestions I've made here.

[kasparsd]

This looks good! I'll merge this in and some linter checks to find any other PHP8 compatibility issues.

[iandunn]

WPCS still requires longhand array syntax, so that's the code suggestions I've made here.

There was a proposal to change to the short syntax, but the decision was to keep the long one.