CI: update for upcoming moodle-plugin-ci

[PhilippImhof]

Next minor version of moodle-plugin-ci is coming soon and has some changes. This PR changes

• master to main when checking out from the Moodle repository
• remove PHP copy/paste detector (phpcpd) which is deprecated and will be removed in version 5

Taking the opportunity to do some cleanup (remove reference to behat from unit test workflow)

[coveralls]

Pull Request Test Coverage Report for Build 7863362525

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 75.03%

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Totals
Change from base Build 7854277284:	0.0%
Covered Lines:	2524
Relevant Lines:	3364

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💛 - Coveralls