Use case-insensitive matching for Git error "Not a valid object name"

[mdferdousalam]

Fixes #3309

Git is lowercasing the fatal: Not a valid object name error message
to follow its CodingGuidelines. This change makes the string matching
case-insensitive so it works with both the current and future Git versions.

Changes:

• Use .toLowerCase() before checking for the error substring

See: https://lore.kernel.org/git/pull.2052.git.1771836302101.gitgitgadget@gmail.com

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
a58ab78

Please, read and sign the above mentioned agreement if you want to contribute to this project

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}