Refactor PostgreSQL logic

What does this Merge Request do and why?

This MR aims to refactor and improve the PostgreSQL logic to make upcoming relating MRs less complex.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

spec/lib/gdk/postgresql_spec.rb

@@ -171,11 +171,14 @@ RSpec.describe GDK::Postgresql do
     end
   end
 
-  def stub_pg_version_file(version = nil, exists: true)
+  def stub_pg_data_dir
     allow(config).to receive(:postgresql).and_return(postgresql_config)
     allow(pg_data_dir).to receive(:join).with('PG_VERSION').and_return(pg_version_file)
-    allow(pg_version_file).to receive(:exist?).and_return(exists)
+  end
 
+  def stub_pg_version_file(version = nil, exists: true)
+    stub_pg_data_dir
+    allow(pg_version_file).to receive(:exist?).and_return(exists)
     allow(pg_version_file).to receive(:read).and_return(version) if version
   end
 end