Revert TS4 and Firestore 5.x updates

[lahirumaramba]

• Reverting TS4 and Firestore 5.x updates.
• Our release process caught that upgrading to TS4 (which is required for Firestore 5.x) breaks TS backwards compatibility.
• We probably have to include the TS and Firestore upgrades in a major version release and require more time to plan the proper next steps.
• For now, I am reverting the updates ((fix): Add new Firestore types to the firestore module #1554 , chore: Update Typescript to 4.x #1541 , feat(firestore): Version update @google-cloud/firestore to 5.x #1525). This will allow us to cut a patch release with node-forge update (build(deps): bump node-forge from 0.10.0 to 1.0.0 #1533).