Use enum-typed PG versions (#12317)

This makes it possible for the compiler to validate that a match block
matched all PostgreSQL versions we support.

## Problem
We did not have a complete picture about which places we had to test
against PG versions, and what format these versions were: The full PG
version ID format (Major/minor/bugfix `MMmmbb`) as transfered in
protocol messages, or only the Major release version (`MM`). This meant
type confusion was rampant.

With this change, it becomes easier to develop new version-dependent
features, by making type and niche confusion impossible.

## Summary of changes
Every use of `pg_version` is now typed as either `PgVersionId` (u32,
valued in decimal `MMmmbb`) or PgMajorVersion (an enum, with a value for
every major version we support, serialized and stored like a u32 with
the value of that major version)

---------

Co-authored-by: Arpad Müller <arpad-m@users.noreply.github.com>

libs/wal_decoder/benches/bench_interpret_wal.rs

@@ -10,7 +10,7 @@ use futures::StreamExt;
 use futures::stream::FuturesUnordered;
 use pageserver_api::shard::{ShardIdentity, ShardStripeSize};
 use postgres_ffi::waldecoder::WalStreamDecoder;
-use postgres_ffi::{MAX_SEND_SIZE, WAL_SEGMENT_SIZE};
+use postgres_ffi::{MAX_SEND_SIZE, PgMajorVersion, WAL_SEGMENT_SIZE};
 use pprof::criterion::{Output, PProfProfiler};
 use remote_storage::{
     DownloadOpts, GenericRemoteStorage, ListingMode, RemoteStorageConfig, RemoteStorageKind,
@@ -115,7 +115,7 @@ struct BenchmarkData {
 
 #[derive(Deserialize)]
 struct BenchmarkMetadata {
-    pg_version: u32,
+    pg_version: PgMajorVersion,
     start_lsn: Lsn,
 }