Stabilise `base` 0.6, `fcos` 1.6 on `ignition` 3.5.0 spec

[travier]

Bumping spec versions

This checklist describes bumping the Ignition spec version, base version, and distro versions. If your scenario is different, modify to taste.

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]

Done with #566