Migrate Secrets Analyzer to use `git` and `dir` modes

Problem to solve

In Gitleaks v8.19.0, the detect mode, used by the secrets analyzer was deprecated. The detect mode still works but is no longer documented in gitleaks help.

It's unclear in which version of Gitleaks the detect mode will be removed.

When the detect mode is removed, the Secrets Analyzer will no longer be able to be upgraded to the latest version of Gitleaks.

Proposal

Update the Secrets Analyzer to use the new git and dir modes.

/cc @amarpatel @abellucci