Stabilize OCP 4.14 spec

[travier]

Bumping spec versions

Bump distro version

• Rename config/distro/vD_exp to config/distro/vD and update package statements. Update imports.
• Drop -experimental from init() in config/config.go.
• Drop -experimental from examples in docs/.
• Copy config/distro/vD to config/distro/vD+1_exp.
• Update package statements in config/distro/vD+1_exp. Bump its base dependency to base/vB+1_exp.
• Import config/vD+1_exp in config/config.go and add distro C+1-experimental to init().

Update docs

• Update internal/doc/main.go to add the new stable spec and reference the new experimental spec in generate().
• Run generate to regenerate spec docs.
• Update docs/specs.md.
• Update docs/upgrading-*.md for the new spec version. Copy the relevant section from Ignition's doc/migrating-configs.md, convert the configs to Butane configs, convert field names to snake case, and update wording as needed. Add subsections for any new Butane-specific features.
• Note the stabilization in docs/release-notes.md, following the format of previous stabilizations. Drop the -exp version suffix from any notes for the upcoming release.

[travier]

We might want to revert 4bd006e from #456 as it does not work with the MCO in 4.14.

[travier]

And we might need to stay on Ignition spec 3.4.0 / the latest supported by the MCO.

[travier]

#490

[travier]

On top of the existing mapping that we have here between Butane and Ignition spec versions, it would be nice to get a doc update in https://github.com/openshift/machine-config-operator/tree/master/docs to add a mapping between Ignition specs and the branch in which they are supported / used as default internal spec in the MCO.

• https://github.com/openshift/machine-config-operator/blob/5b821a279c88fee1cc1886a6cf1ec774891a2258/pkg/controller/common/helpers.go#L609
• OCPBUGS-8447: MCO-496: Support ignition versions 3.3 + 3.4 but keep version 3.2 as default  openshift/machine-config-operator#3576
• MCO-588: Update ignition spec to 3.4, disallow ignition KernelArguments for now  openshift/machine-config-operator#3814