postgresql_11: remove

As described in the release lifecycle docs from postgresql[1], v11 will
stop receiving fixes as of Nov 9 2023. This means it's EOL throughout
the entire lifetime of 23.11, so let's drop it now.

A lot of examples are also referencing postgresql_11. Where it's
sensible, use postgresql_15 as example now to avoid confusion.

This is also handy because the LLVM 16 fix for postgresql is not
available for postgresql 11 ;-)

[1] https://www.postgresql.org/support/versioning/

nixos/modules/services/backup/postgresql-wal-receiver.nix

@@ -7,7 +7,7 @@ let
     options = {
       postgresqlPackage = mkOption {
         type = types.package;
-        example = literalExpression "pkgs.postgresql_11";
+        example = literalExpression "pkgs.postgresql_15";
         description = lib.mdDoc ''
           PostgreSQL package to use.
         '';
@@ -124,7 +124,7 @@ in {
         example = literalExpression ''
           {
             main = {
-              postgresqlPackage = pkgs.postgresql_11;
+              postgresqlPackage = pkgs.postgresql_15;
               directory = /mnt/pg_wal/main/;
               slot = "main_wal_receiver";
               connection = "postgresql://user@somehost";