Build/CI: Bring back (C) header checks

### Describe the bug

#1562 removed the check for and automatic application of (C) headers.

It's too easy to have the wrong (C) header in files. Checks in CI and automatic application help a lot.

Spotless offers ways for what's mentioned on #1562 - for example `targetExcludeIfContentContains/Regex`.

### To Reproduce

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### Actual Behavior

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### Expected Behavior

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### Additional context

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### System information

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Start by reviewing issue #1562 and the repository's current Spotless configuration to locate where header checks were removed. Compare Spotless's targetExcludeIfContentContains/Regex options with the prior behavior; done means CI checks and automatic (C) header application are restored without affecting intended exclusions.

Written by the indexing model from the issue text.