Remove Spring from the project

IMHO, Spring no longer provides benefits in this project and:

- Spring was already disabled in the test environment since commit e4e0cb5d47
- Rails removed Spring as a default installation option in 2021 [1]

[1] PR #42997 from https://github.com/rails/rails/

Gemfile.lock

@@ -661,7 +661,6 @@ GEM
       version_gem (>= 1.1.8, < 3)
     social-share-button (1.2.4)
       coffee-rails
-    spring (4.2.1)
     sprockets (4.2.1)
       concurrent-ruby (~> 1.0)
       rack (>= 2.2.4, < 4)
@@ -826,7 +825,6 @@ DEPENDENCIES
   simplecov-lcov (~> 0.8.0)
   sitemap_generator (~> 6.3.0)
   social-share-button (~> 1.2.4)
-  spring (~> 4.2.1)
   sprockets (~> 4.2.1)
   sprockets-rails (~> 3.5.2)
   turbolinks (~> 5.2.1)