Revert "Bump pg from 1.4.3 to 1.5.4"

We were getting warnings due to an incompatibility between pg 1.5.x and
Active Record. I thought it was fixed in Active Record 6.1.x, but it
isn't, so we're using pg 1.4.x until we upgrade to Rails 7.

The warning was:

```
PG::Coder.new(hash) is deprecated. Please use keyword arguments instead!
Called from
activerecord-6.0.6.1/lib/active_record/connection_adapters/postgresql_adapter.rb:883:in
`new'
```

This reverts commit c03f4243f.

Gemfile.lock

@@ -406,7 +406,7 @@ GEM
     parser (3.2.2.3)
       ast (~> 2.4.1)
       racc
-    pg (1.5.4)
+    pg (1.4.3)
     pg_search (2.3.6)
       activerecord (>= 5.2)
       activesupport (>= 5.2)
@@ -727,7 +727,7 @@ DEPENDENCIES
   omniauth-rails_csrf_protection (~> 1.0.1)
   omniauth-twitter (~> 1.4.0)
   paranoia (~> 2.6.2)
-  pg (~> 1.5.4)
+  pg (~> 1.4.3)
   pg_search (~> 2.3.6)
   pronto (~> 0.11.1)
   pronto-erb_lint (~> 0.1.6)