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/

bin/spring

@@ -1,17 +0,0 @@
-#!/usr/bin/env ruby
-
-# This file loads spring without using Bundler, in order to be fast.
-# It gets overwritten when you run the `spring binstub` command.
-
-unless defined?(Spring)
-  require "rubygems"
-  require "bundler"
-
-  lockfile = Bundler::LockfileParser.new(Bundler.default_lockfile.read)
-  spring = lockfile.specs.find { |spec| spec.name == "spring" }
-  if spring
-    Gem.use_paths Gem.dir, Bundler.bundle_path.to_s, *Gem.path
-    gem "spring", spring.version
-    require "spring/binstub"
-  end
-end