Use go 1.20 in go.mod

Future commit will be using bytes.Clone which was implemented in Go
1.20.

Also update README.md to reflect that minimum supported Go version is
1.21. But only requiring Go 1.20 in go.mod to avoid needlessly breaking
old Go when it still works.

README.md

@@ -92,7 +92,7 @@ See the presentation at Golang Estonia, [PGX Top to Bottom](https://www.youtube.
 
 ## Supported Go and PostgreSQL Versions
 
-pgx supports the same versions of Go and PostgreSQL that are supported by their respective teams. For [Go](https://golang.org/doc/devel/release.html#policy) that is the two most recent major releases and for [PostgreSQL](https://www.postgresql.org/support/versioning/) the major releases in the last 5 years. This means pgx supports Go 1.20 and higher and PostgreSQL 12 and higher. pgx also is tested against the latest version of [CockroachDB](https://www.cockroachlabs.com/product/).
+pgx supports the same versions of Go and PostgreSQL that are supported by their respective teams. For [Go](https://golang.org/doc/devel/release.html#policy) that is the two most recent major releases and for [PostgreSQL](https://www.postgresql.org/support/versioning/) the major releases in the last 5 years. This means pgx supports Go 1.21 and higher and PostgreSQL 12 and higher. pgx also is tested against the latest version of [CockroachDB](https://www.cockroachlabs.com/product/).
 
 ## Version Policy
 

go.mod

@@ -1,6 +1,6 @@
 module github.com/jackc/pgx/v5
 
-go 1.19
+go 1.20
 
 require (
 	github.com/jackc/pgpassfile v1.0.0