ci: test on PostgreSQL 13 and document it as the minimum

[lesnik512]

Closes #216. For 0.15.0: the documented minimum moves from PostgreSQL 12 to 13, and CI now tests on 13.

Why

outbox_autovacuum_ddl always sets autovacuum_vacuum_insert_scale_factor and autovacuum_vacuum_insert_threshold, and both arrived in PostgreSQL 13. On 12 the ALTER TABLE fails with "unrecognized parameter", and validate_schema(check_autovacuum=True) can never pass. The docs said 12+, and CI only ran 17, so nothing caught this. PostgreSQL 12 has been end of life since November 2024.

I ran the full suite locally against both versions, swapping only the Postgres image:

server	result
PostgreSQL 12	3 failed, 675 passed: the three test_validate_schema_autovacuum_ok_* tests
PostgreSQL 13.23	678 passed

No other test depends on 13, so nothing besides the autovacuum options needs it.

Changes

• CI: the existing floors job now runs on postgres:13 instead of 17. That job already installs the lowest declared dependency versions, so it now tests the lowest supported server too, and the pytest matrix stays on 17 with the newest dependencies. No new job. floors skips the weekly scheduled run, so 13 is covered on every pull request and every push to main.
• Local check of that combination: with the floor dependency set (SQLAlchemy 2.0.0, faststream 0.7.6, asyncpg 0.29.0, alembic 1.13.0) against PostgreSQL 13.23, all 678 tests pass on Python 3.11 and on 3.12. CI covers 3.13 and 3.14, which resolve their own floors.
• Docs: installation.md now says PostgreSQL 13+ and why. alembic.md says the insert-triggered pair needs 13 or later.
• No code change.

Release note (0.15.0)

PostgreSQL 13 is now the minimum supported version (12 reached end of life in November 2024). outbox_autovacuum_ddl never worked on 12, and CI now tests on 13 as well as 17.

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	1081	1.000	6.67	909	242	5000	5000	10000
consumer/w1/b100	1341	1.000	7.10	1144	243	5000	5000	10000
consumer/w2/b10	1283	1.000	6.69	921	242	5000	5000	10000
consumer/w2/b100	1766	1.000	7.10	1144	243	5000	5000	10000
consumer/w4/b10	1279	1.000	6.69	922	242	5000	5000	10000
consumer/w4/b100	1815	1.000	7.14	1179	263	5000	5000	10000
consumer/w1/b100/tfbs100	5522	0.010	6.07	1119	244	5000	5000	10000
consumer/w1/b100/interval1-10	1310	1.000	7.10	1144	243	5000	5000	10000
producer/w1/b100	3107	0.000	3.04	585	1	0	0	0
producer/w1/b100/ppt1	746	0.000	4.04	626	0	0	0	0

Gated (fails the build): delete_calls + tuple counters (upd/del/ins) + the producer's insert_calls, exact; select_calls and other_calls within +2; wal_records within a 10% band; fetch_calls within 2x. msg/s, WAL bytes and total calls are informational (timing/FPI noise).