[CI] Commit checker accepts canonical Co-authored-by trailers

## Affected revision

`52b3ab0606d14300ba30a6a46c155c5892a06381` on `jdk17u-target8`.

## Problem

The commit-message checker intends to reject autogenerated co-author trailers, but `check_forbidden_lines` performs a case-sensitive prefix comparison against only:

```text
Co-Authored-By:
```

The canonical Git trailer spelling `Co-authored-by:` therefore passes.

## Minimal reproduction

```powershell
$env:PYTHONIOENCODING='utf-8'
"[Hotspot] Fix`n`nCo-authored-by: Aime " |
python .github/scripts/check_commit.py --stdin
```

Current result:

```text
Commit: [Hotspot] Fix
✅ OK
--- Result: 1/1 passed ---
```

The existing unit test covers only the exact `Co-Authored-By:` casing.

## Impact

A commit can contain the trailer that the repository policy explicitly intends to forbid while still passing the required pull-request commit check.

Git trailer tokens are conventionally treated without case significance, so casing should not bypass this policy. The same normalization should be applied consistently to every entry in `FORBIDDEN_LINES`.

## Expected behavior

`Co-Authored-By:`, `Co-authored-by:`, and other casing variants should all be rejected, with regression coverage for the canonical spelling.

I checked the complete issue/PR inventory and relevant script history; no equivalent report or pending fix was found.

Start with .github/scripts/check_commit.py, focusing on check_forbidden_lines and FORBIDDEN_LINES, then inspect the existing unit test for the exact Co-Authored-By: casing. Reproduce the canonical Co-authored-by: example with the --stdin entry point and add regression coverage; done means all casing variants are rejected while the existing checks still pass.

Written by the indexing model from the issue text.