chore: references to "oauth2client" master branch

[nicain]

During the master --> main migration, I noticed references that point to the master branch of https://github.com/googleapis/oauth2client

After that repo is migrated, we will need to circle back to this repo (and potentially others) and fix these references

• CHANGELOG.md

xref: googleapis/google-cloud-python#10579

[busunkim96]

I believe we'd have to un-archive the oauth2client repo it to change master -> main. Since we don't intend to do future development on oauth2client I would prefer not to.

CC @danoscarmike @JustinBeckwith

[JustinBeckwith]

Yeah no reason to go messing with archived repos. Please leave that as is.