fix(manager/gradle): only accept known format for optional classifiers

[Churro]

Changes

Fixes a regression introduced by allowing optional dependency classifiers with PR #18469.

Unfortunately, there is no strict format on how a valid classifier should look like. According to Maven, it's an arbitrary string. Gradle parses any string, if only the number of separators matches.

The check added in this PR restricts possible classifiers to "reasonable" values, i.e., analogous to how artifactId or groupId look like. This filters obviously invalid classifiers like URLs. The rationale here is: better allow only classifiers with good confidence, than not to extract dependency definitions with classifiers at all.

Context

Closes #18513

Documentation (please check one with an [x])

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How I've tested my work (please tick one)

I have verified these changes via:

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Test repo: renovate-demo/18513-renovate-gradle-properties-bug#1

[renovate-release]

🎉 This PR is included in version 33.2.2 🎉

The release is available on:

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• 33.2.2

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