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

@@ -109,7 +109,6 @@ group :development do
   gem "rubocop-rspec", "~> 3.4.0", require: false
   gem "rubocop-rspec_rails", "~> 2.30.0", require: false
   gem "rvm1-capistrano3", "~> 1.4.0", require: false
-  gem "spring", "~> 4.2.1"
   gem "web-console", "~> 4.2.1"
 end