Normalize repository owner casing to fix GitHub PR integration

[bradly0cjw]

PR Description

Close #5494

Normalizes the repository owner to lowercase during the PR mapping.
This ensures that PR icons and integration features work correctly even when the local git remote URL casing differs from the official repository casing on GitHub.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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🟢 Metrics 1 duplication

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[stefanhaller]

Makes sense, thanks.