Replace Gitleaks as our secret scanner

**Is your feature request related to a problem? Please describe.**
Gitleaks has been superseded by other secret detection tools, which are reported to have lower false positive rates.

**Describe the solution you'd like**
Gitleaks is replaced by a more modern secret scanner, which should be be configurable through the git proxy config.

Start by locating the Gitleaks integration and the git proxy configuration entry point. Compare a modern secret scanner that offers lower false-positive rates, then verify that the replacement is configurable through the proxy config and that secret detection still works as expected.

Written by the indexing model from the issue text.