Automate the discovery/usage of latest crossplane release in test workflows

As per the discussion in https://github.com/crossplane/crossplane/pull/3219#issuecomment-1205750668, it would be good to add automation to our testing workflows so we no longer hard code the release branch and crossplane version. In that PR, we added a manual step to the release process to update our test workflows to use the newest release branch and version. It would be better if this was automated instead.

Some examples in our test workflows where we are hard coding branch and version numbers:

• https://github.com/crossplane/test/blob/40a60b1698876ca2b89fd4f0c4dea61675a6ea20/.github/workflows/crossplane-upgrade.yml#L21
• https://github.com/crossplane/test/blob/40a60b1698876ca2b89fd4f0c4dea61675a6ea20/.github/workflows/crossplane-upgrade.yml#L36
• https://github.com/crossplane/test/blob/40a60b1698876ca2b89fd4f0c4dea61675a6ea20/.github/workflows/provider-upgrade.yml#L35

There are others too - we should automate all of them.