Automate Git tagging on new version cut

[ndrewr]

Currently we do this manually via our local CLI after deploying to a Prod Org (means validation testing passed).

We can have this done automatically in a Github Action, perhaps on the merge event into master branch.

Also note the suggested "Best Practice" when working with 2nd-Gen-Packaging:
https://developer.salesforce.com/docs/atlas.en-us.sfdx_dev.meta/sfdx_dev/sfdx_dev_dev2gp_plan_best_practices.htm

Include the --tag option when you use the package:version:create and package:version:update commands. This option helps you keep your version control system tags in sync with specific package versions.