Remove govuk_admin_template gem and configuration

We no longer need this gem as we aren't using the govuk_admin_template
any more. I've read the setup instructions[1] for this (deprecated)
gem and reversed the configuration mentioned in that README.

[1] https://github.com/alphagov/govuk_admin_template/blob/5fc0de2dd570e12b5d816261cbe4a4bc18752845/README.md

Gemfile

@@ -33,7 +33,6 @@ gem "whenever"
 
 # GDS Gems
 gem "gds-api-adapters"
-gem "govuk_admin_template"
 gem "govuk_app_config"
 gem "govuk_publishing_components"
 gem "govuk_sidekiq"

Gemfile.lock

@@ -74,8 +74,6 @@ GEM
     ancestry (4.3.3)
       activerecord (>= 5.2.6)
     ast (2.4.2)
-    autoprefixer-rails (10.4.16.0)
-      execjs (~> 2)
     bcrypt (3.1.20)
     better_errors (2.10.1)
       erubi (>= 1.0.0)
@@ -85,9 +83,6 @@ GEM
       debug_inspector (>= 0.0.1)
     bootsnap (1.17.1)
       msgpack (~> 1.2)
-    bootstrap-sass (3.4.1)
-      autoprefixer-rails (>= 5.2.1)
-      sassc (>= 2.0.0)
     brakeman (6.1.1)
       racc
     browser (5.3.1)
@@ -159,11 +154,6 @@ GEM
     google-protobuf (3.25.2-x86_64-linux)
     googleapis-common-protos-types (1.11.0)
       google-protobuf (~> 3.18)
-    govuk_admin_template (6.10.0)
-      bootstrap-sass (~> 3.4)
-      jquery-rails (~> 4.3)
-      plek (>= 2.1)
-      rails (>= 6)
     govuk_app_config (9.8.1)
       logstasher (~> 2.1)
       opentelemetry-exporter-otlp (>= 0.25, < 0.27)
@@ -210,10 +200,6 @@ GEM
     http-form_data (2.3.0)
     i18n (1.14.1)
       concurrent-ruby (~> 1.0)
-    jquery-rails (4.6.0)
-      rails-dom-testing (>= 1, < 3)
-      railties (>= 4.2.0)
-      thor (>= 0.14, < 2.0)
     json (2.7.1)
     jsonpath (1.1.5)
       multi_json
@@ -712,7 +698,6 @@ DEPENDENCIES
   doorkeeper
   factory_bot_rails
   gds-api-adapters
-  govuk_admin_template
   govuk_app_config
   govuk_publishing_components
   govuk_sidekiq