logical,txn: allowlist and version-gate DROP COLUMN for LDR

[DarrylWong]

Summary

Allow the declarative schema changer to run the supported DROP COLUMN migration on transactional LDR tables, after all required decoder and index-rewrite correctness work is cluster-wide.

Initial supported scope

Support dropping a nullable, non-primary-key column that is not involved in:

• Secondary indexes
• Unique, foreign-key, or check constraints
• Computed, default, or on-update expressions
• Multiple column families
• ALTER PRIMARY KEY
• Compound ALTER TABLE statements or DROP COLUMN ... CASCADE

Scope

• Extend the LDR DDL allowlist for the narrow DROP COLUMN operation.
• Require every LDR job referencing the table to permit online schema changes.
• Keep ON_SCHEMA_CONFLICT and TRUNCATE EXTRA COLUMNS as separate safety controls: the former enables schema skew; the latter authorizes dropping non-NULL source-only data.
• Reject unsupported dependency/index cases before changing the descriptor, with actionable errors.
• Add a cluster-version gate that becomes active only after live-row metadata, tombstones, temporary-index merge metadata, and decoder truncation support are present on every node.
• Verify source and destination schema replanning across the drop boundary.
• Add focused SQL/integration tests for allowed and rejected forms.

Done when

The supported DROP operation completes while LDR continues through temporary schema skew, unsupported forms are rejected before mutation, and mixed-version clusters cannot enter a state older nodes handle incorrectly.

Dependencies

Depends on all three index-backfill correctness tickets, per-table TRUNCATE EXTRA COLUMNS, and the policy-gated column-change work in cockroachlabs/cockroach#5064.

Design: https://docs.google.com/document/d/1FRb0lVEKAZGOGW0csPw5HBtf5_FqSPBoP-RcW_pnVdM/edit

Jira issue: CRDB-68658

Epic CRDB-68660

[blathers-crl]

cc @cockroachdb/cdc