fix: change maven coords

GPC projects should publish to `io.github.gpc`.

Author: matrei

README.md

@@ -53,8 +53,8 @@ In `build.gradle` add the following dependencies to install the plugin:
 ```groovy
 dependencies {
     //...
+    implementation 'io.github.gpc:grails-postgresql-extensions:<version>'
     implementation 'org.apache.grails:grails-data-hibernate5'
-    implementation 'org.grails.plugins:grails-postgresql-extensions:<version>'
     //...
 }
 ```

plugin/build.gradle

@@ -4,7 +4,7 @@ plugins {
 }
 
 version = projectVersion
-group = 'org.grails.plugins'
+group = 'io.github.gpc'
 
 grailsPublish {
     artifactId = 'grails-postgresql-extensions'