sql: graduate the bounded staleness isolation level

[antiguru]

Motivation

bounded staleness <duration> shipped in v26.29 behind
enable_bounded_staleness_isolation, which has been defaulting on since. The
level is documented as public preview and still carries the gating machinery
that a preview needs. This graduates it: the flag goes away and the docs drop
the preview annotation, so bounded staleness is a supported part of the
transaction_isolation surface.

Description

• Removes enable_bounded_staleness_isolation from the feature-flag table.
  check_transaction_isolation_feature_flag now gates only strong session serializable, which is still flagged behind enable_session_timelines. The
  shared check and the session-default scrub in command_handler therefore
  stay in place; only the bounded staleness arm and the comment's example
  change.
• An environment that persisted the removed parameter via ALTER SYSTEM is
  unaffected: an unknown system parameter in catalog storage is warned about
  and ignored at startup (catalog/apply.rs, catalog/open.rs).
• Drops {{< public-preview />}} and the Public preview. cell from the
  isolation-level reference. The {{< if-released "v26.29" >}} guards are kept,
  since older Self-Managed versions still lack the level entirely.
• Parallel workload no longer gates BoundedStalenessReadAction on the flag or
  flips the flag, and the flag leaves the LaunchDarkly consistency allowlist
  (it never had an LD flag, so it was listed under KNOWN_MISSING_FROM_LD).

Not included: a release-note entry in doc/user/content/releases/_index.md.
That file has no section for the in-flight version, so the GA note belongs with
the release-notes commit for whichever release this lands in.

Verification

• test/sqllogictest/bounded_staleness.slt loses the feature-flag sections and
  keeps the role-default coverage: ALTER ROLE ... SET of the level, and a
  fresh session picking it up as its starting isolation.
• test/testdrive/bounded-staleness.td no longer enables the flag; the
  happy-path coverage is otherwise unchanged.
• The mz-sql unit test for the shared isolation gate now exercises strong session serializable (rejected with the flag off, accepted with it on) and
  asserts bounded staleness passes ungated.

Release note: This release makes the bounded staleness isolation level
generally available.

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Generated by Claude Code