config: Do not use semantic commit messages for Gitaly

By default, Renovate will try to auto-detect whether repositories use semantic commit messages. If this auto-detection detects that a repo does, then it will start using Angular-style commit prefixes in the form of "chore(deps):".

While Gitaly does use semantic scopes in its commit messages, these scopes aren't Angular-style scopes at all. So let's disable semantic commits and instead use a static prefix based on the language we're about to update packages for.