gemspec: remove open ended dependency

Got
```
WARNING:  open-ended dependency on pg (>= 1.2) is not recommended
  if pg is semantically versioned, use:
    add_runtime_dependency 'pg', '~> 1.2'
```

so applying the suggestion.

Author: otan

activerecord-cockroachdb-adapter.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/cockroachdb/activerecord-cockroachdb-adapter"
 
   spec.add_dependency "activerecord", "~> 6.1"
-  spec.add_dependency "pg", ">= 1.2"
+  spec.add_dependency "pg", "~> 1.2"
   spec.add_dependency "rgeo-activerecord", "~> 7.0.0"
 
   # Prevent pushing this gem to RubyGems.org. To allow pushes either set the 'allowed_push_host'