Deprecating the UPE preview feature flag and removing its usages

[wjrosa]

Changes proposed in this Pull Request:

In this PR, I am deprecating the is_upe_preview_enabled method and removing the actual feature flag (_wcstripe_feature_upe). This has not been used for a while, since we deprecated the legacy checkout. I am also updating all parts of the code referencing the is_upe_preview_enabled method.

Testing instructions

Code review. Check if the tests are still passing. Perform some basic smoke-testing.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
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[malithsen]

Code changes look good to me. Tests are failing. It doesn't look like failures are related to the changes in this PR, though we'd still want to figure out why they are failing before merging.

[wjrosa]

@malithsen, it looks like something related to Composer itself or PHPUnit with PHP 7.4. I tried to update the version of the setup-php action to the latest, but that did not fix the issue. I see the same failures on other PRs. I will spend a bit more time trying to understand the root cause later today