Replace substr and strpos with str_starts_with for better readability

[Soean]

Description of the Change

This pull request refactors several string comparison operations to use the more readable and modern str_starts_with() function instead of manual substr or strpos checks. This improves code clarity and consistency when checking for specific string prefixes.

Edit:
str_starts_with was introduced with PHP 8.0 and polyfilled in WordPress 5.9

The plugin minimum is:

• PHP: 7.4
• WordPress: 6.6

So we need to increase the WordPress or PHP minimum to merge the PR.

Changelog Entry

Developer - Replace substr and strpos with str_starts_with for better readability

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.

[jeffpaul]

@Soean seeing this is in draft, so we'd normally hold off reviewing until it comes out of draft by also noting that Darin and I are almost fully focused until mid-April on the https://github.com/wordPress/ai plugin so it may be a while until we get back to this PR, so your patience is appreciated, thanks!