Description of your changes

This PR adds the v2alpha1 version for apiextensions to a few places, so they understand what a v2 XRD (i.e. CompositeResourceDefinition.apiextensions.crossplane.io/v2alpha1) is:

• the apiextensions module setup
• the xpkg linter
• the xpkg parser

This fixes the following scenarios, enabling you to use v2 XRDs in them:

• crossplane xpkg build a configuration package that has a v2 XRD in it
• crossplane xpkg install a configuration package that has a v2 XRD in it

I have tested this with the following commands and the content in this gist: https://gist.github.com/jbw976/c51c8262c4bdd93f6a358f7cadee9a5c

# build a crank binary from local changes in this PR
go install ./cmd/crank

# build and push the xpkg, note this now passes the parser and linter checks
crank xpkg build --package-file=test-package.xpkg
crank xpkg push --package-files=test-package.xpkg ghcr.io/jbw976/v2-xrd:v0.0.1

# run a hack cluster with the local changes in this PR
earthly +hack --SIMULATE_CROSSPLANE_VERSION=v2.0.0-rc.0

# install the xpkg into the hack cluster
crank xpkg install configuration ghcr.io/jbw976/v2-xrd:v0.0.1

With the fixes in this PR, the configuration packages becomes healthy and installed ✅

Fixes #6364

I have:

• Read and followed Crossplane's contribution process.
• Run earthly +reviewable to ensure this PR is ready for review.
• Added or updated unit tests.
• Added or updated e2e tests.
• Linked a PR or a docs tracking issue to document this change.
• Added backport release-x.y labels to auto-backport this PR.
• Followed the API promotion workflow if this PR introduces, removes, or promotes an API.

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