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/postgres_ffi/benches/waldecoder.rs

@@ -4,6 +4,7 @@ use criterion::{Bencher, Criterion, criterion_group, criterion_main};
 use postgres_ffi::v17::wal_generator::LogicalMessageGenerator;
 use postgres_ffi::v17::waldecoder_handler::WalStreamDecoderHandler;
 use postgres_ffi::waldecoder::WalStreamDecoder;
+use postgres_versioninfo::PgMajorVersion;
 use pprof::criterion::{Output, PProfProfiler};
 use utils::lsn::Lsn;
 
@@ -32,7 +33,7 @@ fn bench_complete_record(c: &mut Criterion) {
         let value_size = LogicalMessageGenerator::make_value_size(size, PREFIX);
         let value = vec![1; value_size];
 
-        let mut decoder = WalStreamDecoder::new(Lsn(0), 170000);
+        let mut decoder = WalStreamDecoder::new(Lsn(0), PgMajorVersion::PG17);
         let msg = LogicalMessageGenerator::new(PREFIX, &value)
             .next()
             .unwrap()