Code Modernization: Use str_contains() instead of strpos()

[Soean]

str_contains() was introduced in PHP 8.0 to perform a case-sensitive check indicating if the string to search in (haystack) contains the given substring (needle).

WordPress core includes a polyfill for str_contains() on PHP < 8.0 as of WordPress 5.9.

This PR replaces false !== strpos( ... ) and false === strpos( ... ) with str_contains() in core files, making the code more readable and consistent, as well as better aligned with modern development practices.

Introduced in r60269 and r60939

Trac ticket: https://core.trac.wordpress.org/ticket/65408

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Props soean.

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[github-actions]

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[mukeshpanchal27]

Code Modernization 🚀

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62461
GitHub commit: 8bed69c

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.