chore: release as 2.7.0 (#247)

Yesterday's [release PR for version 2.6.0 ](#236) failed because the PR was trying to release 2.6.0 which was [already released](#231) and [tagged](https://github.com/googleapis/python-monitoring/releases/tag/v2.6.0). This PR tells release-please to change the version to 2.7.0. I've also carried over the release notes from yesterday's release PR PR.




feat: add CreateServiceTimeSeries RPC
fix(deps): drop packaging dependency
fix(deps): require google-api-core >= 1.28.0
fix: Reintroduce deprecated field/enum `ServiceTier` for backward compatibility
docs: list oneofs in docstring
docs: Use absolute link targets in comments

Release-As: 2.7.0