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.

Review the repository's dependency configuration and the closed version-bump PR #468 first, paying particular attention to the rexml dependency mentioned in the issue. Done means the project's dependencies have been brought up to date and the resulting dependency changes are verified without regressions.

Written by the indexing model from the issue text.