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

[jbw976]

As per the discussion in crossplane/crossplane#3219 (comment), 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:

• test/.github/workflows/crossplane-upgrade.yml

  Line 21 in 40a60b1

  ref: release-1.9

• test/.github/workflows/crossplane-upgrade.yml

  Line 36 in 40a60b1

  run: helm repo add crossplane-stable https://charts.crossplane.io/stable && helm repo update && helm install crossplane --namespace crossplane-system crossplane-stable/crossplane --version 1.9.0 --wait

• test/.github/workflows/provider-upgrade.yml

  Line 35 in 40a60b1

  run: helm repo add crossplane-stable https://charts.crossplane.io/stable && helm repo update && helm install crossplane --namespace crossplane-system crossplane-stable/crossplane --version 1.9.0 --wait

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