ci: Remove PHP 5.6 Tests run

[anomiex]

Proposed changes:

We were going to include this removal in a larger update in #34126, but #34127 got merged first and broke this test. So let's do this right away to stop having the failing job in the CI runs.

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Is CI happy?

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