Deprecate setup-local --constraints-only in favour of --no-dbconnect

[rugpanov]

Why

databricks environments setup-local now offers the orthogonal --no-dbconnect
flag, which skips the databricks-connect dependency with behaviour identical to
the older --constraints-only. Two visible spellings for one behaviour is
confusing, but --constraints-only may already live in users' scripts and CI,
so it can't just be removed.

Stacked on #6464 (which introduced --no-dbconnect).

What

Mark --constraints-only deprecated with cobra's MarkDeprecated, which:

• hides it from --help, and
• prints a one-line stderr notice — Flag --constraints-only has been deprecated, use --no-dbconnect instead — once per run when the flag is used.

The flag stays defined and its behaviour is unchanged, so existing callers keep
working. This matches the repo's existing deprecation pattern (--compute-id →
--cluster-id, and the aitools --global/--project flags). Actual removal is
left as a separate, later step.

Backward compatibility

• Default runs are byte-for-byte unchanged.
• --constraints-only still works identically; the only new output is a stderr
  line, so --output json consumers (whose contract is stdout) are unaffected —
  schemaVersion stays 1.

Testing

• Unit (cmd/environments): a new test asserts --constraints-only stays
  defined but is Hidden and carries the use --no-dbconnect instead
  deprecation message.
• Acceptance (acceptance/localenv): regenerated goldens — --constraints-only
  dropped from the help listing; the constraints-only and
  constraints-only-existing runs now show the stderr deprecation line with
  stdout JSON unchanged.
• gofmt, go vet ./cmd/environments, go test ./cmd/environments, and
  go build ./... all clean.

This pull request and its description were written by Isaac.

[github-actions]

Approval status: pending

/acceptance/localenv/ - needs approval

Files: acceptance/localenv/constraints-only-existing/output.txt, acceptance/localenv/constraints-only/output.txt, acceptance/localenv/help/output.txt
Suggested: @anton-107
Also eligible: @rclarey, @misha-db

/cmd/environments/ - needs approval

Files: cmd/environments/sync.go, cmd/environments/sync_test.go
Suggested: @anton-107
Also eligible: @rclarey, @misha-db

General files (require maintainer)

Files: .nextchanges/cli/setup-local-deprecate-constraints-only.md
Based on git history:

• @janniklasrose -- recent work in .nextchanges/cli/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db, @rclarey) can approve all areas.
See OWNERS for ownership rules.}