Refactor string checks to use str_starts_with and str_contains functions

[Soean]

What?

This pull request updates several string handling operations to use more modern and readable PHP string functions. The changes improve code clarity and maintainability by replacing older strpos patterns with str_starts_with and str_contains.

Changelog Entry

Changed - Refactor strpos to str_starts_with and str_contains.

[github-actions]

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Co-authored-by: Soean <soean@git.wordpress.org>
Co-authored-by: justlevine <justlevine@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.44%. Comparing base (7d98557) to head (5789bf7).
⚠️ Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
includes/Abilities/Image/Alt_Text_Generation.php	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #438   +/-   ##
==========================================
  Coverage      68.44%   68.44%           
  Complexity       846      846           
==========================================
  Files             56       56           
  Lines           4095     4095           
==========================================
  Hits            2803     2803           
  Misses          1292     1292           

Flag	Coverage Δ
unit	68.44% <33.33%> (ø)

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[dkotter]

So PHPStan is failing here as it can't find these functions. I think because they don't exist in PHP 7.4, though they are safe for us to use as WordPress polyfills these. Ideally we'll need to find a way to make PHPStan happy here before merging

[justlevine]

Ideally we'll need to find a way to make PHPStan happy here before merging

Done in 5789bf7

Prior art: https://github.com/WordPress/performance/blob/518db24b7606bf7ffda33f0ebba65b5d1e906686/phpstan.neon.dist#L16-L20