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/

config/spring.rb

@@ -1,6 +0,0 @@
-%w[
-  .ruby-version
-  .rbenv-vars
-  tmp/restart.txt
-  tmp/caching-dev.txt
-].each { |path| Spring.watch(path) }