Bump ignition experimental to 3.6.0-experimental

[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.

Stabilize Ignition spec version

• Bump go.mod for new Ignition release and update vendor.
• Update imports. Drop -experimental from Ignition spec versions in */translate_test.go.

Bump base version

• Rename base/vB_exp to base/vB and update package statements. Update imports.
• Copy base/vB to base/vB+1_exp.
• Update package statements in base/vB+1_exp.

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().

Bump Ignition spec version

• Bump Ignition types imports and rename ToIgnI and TestToIgnI functions in base/vB+1_exp. Bump Ignition spec versions in base/vB+1_exp/translate_test.go.
• Bump Ignition types imports in config/distro/vD+1_exp. Update ToIgnI function names, util calls, and header comments to ToIgnI+1.

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.

[prestist]

Closed by #562