Bump Dependency Versions

**Describe the solution you'd like**
Ensure that any dependencies, such as rexml, are up-to-date.

**Describe alternatives you've considered**
We could, perhaps, leave dependencies as-is. Given the time since last update, this is probably not desirable.

**Additional context**
Dependabot opened a [version bump PR](https://github.com/google/GoogleSignIn-iOS/pull/468) many moons ago. I closed it in order to clear out stale branches.