Revert "fix(go): fix Go client to normalize request header keys using…

[wing328]

… Header.Set (#24766)"

This reverts commit 091a02f.

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Summary by cubic

Reverts the Go client header normalization fix so request header keys are no longer canonicalized by Header.Set. The client now assigns header values directly, preserving key casing like api_key.

• Go client template and all generated Go samples switch back to headers[h] = []string{v}.
• Auth tests updated to expect lowercase api_key in dumped request headers.
• Go module version lowered from 1.25 to 1.22, removing now-unused go.sum entries from associated samples.

^{Written for commit c0f11c5. Summary will update on new commits.}