feature request: airgap method for updating Flux2's CRDs

LarryS was having this issue: Helm install failed: unable to build kubernetes objects from release manifest: error validating "": error validating data: [ValidationError(HelmRelease.spec.install): unknown field "crds" in io.fluxcd.toolkit.helm.v2beta1.HelmRelease.spec.install, ValidationError(HelmRelease.spec.upgrade): unknown field "crds" in io.fluxcd.toolkit.helm.v2beta1.HelmRelease.spec.upgrade]

I'll explain what the issue was and that we have a need for a feature request. I suggested 2 days ago that Larry update his version of Flux and he did, but he accidentally did a partial update of flux because he's in a mock airgap env.

The state his cluster was in was: flux system namespace showed the correct latest version of flux. But based on his error message mentioning v2beta1 and TomR mentioning a while back that Flux 0.16.x mentioned they were going to switch from beta api to stable api. the CRDs in his airgap cluster were out of date / on the v2beta1 API version of the CRDs vs the stable api version of the CRDs. Which is where his error came from.

The janky way I helped fix his issue was to:

1. k get deploy -n=flux-system -o yaml > ~/flux.backup.yaml # (because this has the correct version of flux manifests pointing to his air gap registry)
2. vi the flux.backup.yaml and remove references of uid and resourceVersion so that we could restore the file with a kubectl apply -f ~/flux.backup.yaml
3. Airgap manual update the version of his fluxcli binary to 1.16.2
4. Use flux install to have the fluxcli binary generate the CRDs with the stable api version and deploy docker hub / docker.io version of flux images (which were blocked by the airgap environment/didn't work)
5. Since the flux cli fixed the version of the CRDs we were then able to run kubectl apply -f ~/flux.backup.yaml which restored the updated flux that pointed to his airgap registry.
6. Helm install stopped erroring on install.

The feature request: We should have an easier way to do an airgap update of only the CRDs, I'm not sure why but for some reason when we cloned bigbang and ran ./scripts/install_flux.sh we couldn't get it to work airgap + even if it did work it makes too many assumptions about pointing to registry1.dso.mil and being internet connected. (technically this may be a low value feature request that's a 1 off from flux's API's of it's CRD's going from beta to stable and maybe we can just document the workaround? / create a doc to solve this in a cleaner way.)