Refine automated PR workflow process (#2229)

Stop using the update PR description action as is not reliable and this
can't get pulled into release notes anyway.

Disable automerge if there's breaking changes in the schema. Started by
doing this in the PR workflow, but now doing this before even enabling
automerge from the nightly workflow. Currently, we rely on the changelog
label being missing to avoid automerging before review which is *not*
reliable and will break when we move away from the manual changelog
process to automated release notes.

Also fixing up a bunch of warnings from the nightly workflow around
invalid arguments and outdated runtimes.

Example workflow with breaking changes:
https://github.com/pulumi/pulumi-azure-native/actions/runs/4115176481/jobs/7103534228
- Automerge step was skipped
- Resulting PR: #2232

Example run which should also have warnings fixed:
https://github.com/pulumi/pulumi-azure-native/actions/runs/4116003769

Author: danielrbradley