Bump main's version.json to 12.0 (next-major trunk)

[ChrisonSimtian]

Sibling of milestone #13. RFC #267.

What

After release/v11 is cut, bump main's version.json major from 11 → 12. main becomes the v12-prep trunk; release/v11 keeps version.json at 11.

This is what makes the release-branch model honest: NB.GV stops computing 11.0.x heights on the trunk and starts computing 12.0.x ones. Patch heights for v11 are owned by release/v11's commit graph from this point on.

Acceptance criteria

• main's version.json version field is 12.0 (or whatever the next-major shape is — confirm with NB.GV semantics).
• release/v11's version.json is untouched (still 11.0).
• Confirmed via nbgv get-version on both branches that they produce the expected major.
• CHANGELOG note under [Unreleased] — 11.0 → ### Process describing the trunk flip.

Depends on

• release/v11 exists.

Risks

• If a contributor opens a PR targeting main with breaking changes labelled target/v11, the label and the trunk's version drift. PR-creation flow doc needs updating to clarify: target/v11 PRs should target release/v11 after this lands, not main. Coordinate with the documentation issue.