Bump V18 from 18.0.0 to 18.3.0 to fix darwin/arm64 (#166)

The upstream EnterpriseDB macOS binaries for PostgreSQL 18.0.0 were
x86_64-only (missing the arm64 slice), causing "bad CPU type in
executable" on Apple Silicon Macs without Rosetta 2. This was fixed
in EDB's build system (EnterpriseDB/edb-installers#409) and the
18.3.0 binaries ship as proper universal binaries.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: nzoschke

README.md

@@ -32,7 +32,7 @@ following to add the latest release to your project.
 go get -u github.com/fergusstrange/embedded-postgres
 ```
 
-Please note that Postgres 18 & Mac/Darwin builds require [Rosetta 2](https://github.com/fergusstrange/embedded-postgres/blob/cf5b3570ca7fc727fae6e4874ec08b4818b705b1/.circleci/config.yml#L28).
+Please note that Postgres versions before 18.3.0 on Mac/Darwin require [Rosetta 2](https://github.com/fergusstrange/embedded-postgres/blob/cf5b3570ca7fc727fae6e4874ec08b4818b705b1/.circleci/config.yml#L28) on Apple Silicon due to the upstream binaries being x86_64-only. Version 18.3.0+ includes universal binaries that work natively on Apple Silicon.
 
 ## How to use
 
@@ -43,7 +43,7 @@ This library aims to require as little configuration as possible, favouring over
 | Username            | postgres                                        |
 | Password            | postgres                                        |
 | Database            | postgres                                        |
-| Version             | 18.0.0                                          |
+| Version             | 18.3.0                                          |
 | Encoding            | UTF8                                            |
 | Locale              | C                                               |
 | CachePath           | $USER_HOME/.embedded-postgres-go/               |

config.go

@@ -153,7 +153,7 @@ type PostgresVersion string
 
 // Predefined supported Postgres versions.
 const (
-	V18 = PostgresVersion("18.0.0")
+	V18 = PostgresVersion("18.3.0")
 	V17 = PostgresVersion("17.5.0")
 	V16 = PostgresVersion("16.9.0")
 	V15 = PostgresVersion("15.13.0")