closes https://github.com/woodpecker-ci/woodpecker/issues/3071
1. If a secret can be used on PRs, it can also be used on PR close.
2. If no events are set, disallow access to secret. This was different
before, secrets without any event set were allowed for all events.
3. Compare strings instead of patterns.
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Co-authored-by: 6543 <6543@obermui.de>