[Backport 6.53.x] exclude .git from copyright linter

[agent-platform-auto-pr]

Backport 7b0a606 from #33359.

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What does this PR do?

When doing the copyright linting we collect all files matching *.go. If a branch is named with a name ending with .go, like NDMII-3313-integration-metric-stats.go you end up with a file .git/logs/refs/remotes/origin/NDMII-3313-integration-metric-stats.go that we are trying to lint. This PR exclude the .git folder from the linter.

Motivation

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