Bumping grpc core version

[Qi77Qi]

1.28.1 is already being brought in transitively. Forcing it explicitly so that we don't have multiple versions.

Bumping firestore dependency as well because it's bringing in grpc 1.12.0, which is causing some issue if we were using this in google2 as dependency in ammonite. This shouldn't cause any breaking change for caller of GoogleFirestoreService. Within the library usage of firestore lib, there's one deprecation, which shouldn't be an issue since we set it to true previously.

     * @deprecated This setting now defaults to true and will be removed in a future release. If you
     *     are already setting it to true, just remove the setting. If you are setting it to false,
     *     you should update your code to expect {@link com.google.cloud.Timestamp Timestamps}
     *     instead of {@link java.util.Date Dates} and then remove the setting.
     */
    @Deprecated
    @Nonnull
    public Builder setTimestampsInSnapshotsEnabled(boolean value)

cc @dvoet

PR checklist

• For each library you've modified here, decide whether it requires a major, minor, or no version bump. (Click here for guidance)

If you're doing a major or minor version bump to any libraries:

• Bump the version in project/Settings.scala createVersion()
• Update CHANGELOG.md for those libraries
• I promise I used @deprecated instead of deleting code where possible

In all cases:

• Replace the appropriate version hashes in README.md and the CHANGELOG.md for any libs you changed with TRAVIS-REPLACE-ME to auto-update the version string
• Get two thumbsworth of PR review
• Verify all tests go green (CI and coverage tests)
• Squash commits and merge to develop
• Delete branch after merge