Stabilize OCP 4.14 spec

[travier]

Bumping spec versions

Bump distro version

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

Update docs

• [x] Update internal/doc/main.go to add the new stable spec and reference the new experimental spec in generate().
• [x] Run generate to regenerate spec docs.
• [x] Update docs/specs.md.
• [x] 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.
• [x] 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 https://github.com/coreos/butane/commit/4bd006e3f00eae89f9555e986b331fce8fdc2882 from https://github.com/coreos/butane/pull/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]

https://github.com/coreos/butane/issues/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
• https://github.com/openshift/machine-config-operator/pull/3576
• https://github.com/openshift/machine-config-operator/pull/3814