Small refactor: versioner middleware

[ericproulx]

Summary\n\n- Define methods dynamically.\n- Evaluate error_headers at initialization.\n- Simplify potential_versions.\n\nThis PR introduces a small refactor to the versioner middleware for improved clarity and maintainability.

[grape-bot]

	2 Warnings
⚠️	There're library changes, but not tests. That's OK as long as you're refactoring existing code.
⚠️	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#14](https://github.com/ericproulx/grape/pull/14): Small refactor: versioner middleware - [@ericproulx](https://github.com/ericproulx).

Generated by 🚫 Danger

[coveralls]

Pull Request Test Coverage Report for Build 16250291989

Details

• 21 of 21 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 98.192%

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Totals
Change from base Build 16236064626:	-0.004%
Covered Lines:	3367
Relevant Lines:	3429

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💛 - Coveralls