Replace Psalm with PHPstan

[AlexP11223]

Replaced Psalm with PHPstan.

This PR includes many PHPStan error fixes, but most of them are quite simple, such as adding missing type checks, updating phpDocs, or skipping errors when makes sense (e.g. was skipped before with Psalm). I think many of them are in the old code (before Psalm baseline) that was not touched often, so hopefully it will not result in many merge conflicts.

Notable changes:

• Removed return after wp_send_* (it calls die/wp_die)
• Replaced is_a with instanceof (is_a is supposed to accept only objects)
• Added missing ( ) for ??, like 'value: ' . $a[123] ?? 'none'

For reviewing, it's probably easier to go commit by commit, since they are somewhat grouped by fix type.

PHPStan level is set to 8, and generated baseline, which mainly consists of things like missing array type phpDoc, missing return types.

The inpsyde/wp-stubs package requires PHP 8 to avoid parsing error, so I updated the default version in DDEV.

The GHA workflow was updated in #4070

[github-actions]

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What's included:

• ✅ WordPress (latest)
• ✅ WooCommerce (latest)
• ✅ PayPal Payments plugin v3.4.0-pr4069-21950312850-g0672f29 (built from this PR)

Login credentials:

• Username: admin
• Password: password

Plugin Details:

• Version: 3.4.0-pr4069-21950312850-g0672f29
• Commit: 0672f29
• Artifact: woocommerce-paypal-payments-3.4.0-pr4069-21950312850-g0672f29

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