Stabilise MSC4156: `server_name` -> `via` #3010

Job	Run time
check-lockfile	4s
changes	2s
lint-crlf	3s
lint-newsfile	35s
Typechecking	1m 40s
check-schema-delta	45s
lint	18s
check-sampleconfig	47s
lint-pydantic	39s
lint-clippy	0s
lint-clippy-nightly	0s
lint-readme	0s
lint-rustfmt	0s
linting-done	3s
calculate-test-jobs	4s
trial-olddeps	7m 53s
complement (monolith, SQLite)	12m 54s
portdb (3.8, 11)	3m 5s
complement (monolith, Postgres)	13m 42s
portdb (3.11, 15)	3m 0s
complement (workers, Postgres)	20m 58s
cargo-bench	0s
cargo-test	0s
trial-pypy	0s
trial (3.8, sqlite, all)	5m 29s
trial (3.8, postgres, 11, all)	12m 43s
sytest (focal)	8m 43s
sytest (focal, postgres)	10m 24s
trial (3.8, sqlite)	5m 11s
sytest (focal, multi-postgres, workers)	13m 16s
sytest (focal, multi-postgres, workers, asyncio)	19m 7s
export-data	2m 22s
tests-done	3s
	2h 23m 50s

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