* Validate CRDs against latest Kubernetes versions

Add Kubernetes v1.19 and v1.20 series images, and consolidate the job
into a single file to reduce repetition.
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Ignore job if the changes are only site/design
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Fix codespell error
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Cache Velero binary for reuse on workers

This will cache the Velero binary based on the PR number and a SHA256 of
the generated binary.

This way, the runners testing each version of Kubernetes do not need to
build it independently.
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Fix GitHub event access
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Wrap output path in quotes
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Move code checkout to build step
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Also cache go modules
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Fix syntax issues
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Download cached binary on each node
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

* Use cached go modules on main CI
Signed-off-by: Nolan Brubaker <brubakern@vmware.com>