Auto merge of rust-lang#115898 - onur-ozkan:config-change-tracking, r…

…=Mark-Simulacrum

bootstrap major change detection implementation

The use of `changelog-seen` and `bootstrap/CHANGELOG.md` has not been functional in any way for many years. We often do major/breaking changes but never update the changelog file or the `changelog-seen`. This is an alternative method for tracking major or breaking changes and informing developers when such changes occur.

Example output when bootstrap detects a major change:
![image](https://github.com/rust-lang/rust/assets/39852038/ee802dfa-a02b-488b-a433-f853ce079b8a)

test.sh

@@ -214,7 +214,7 @@ function setup_rustc() {
     rm config.toml || true
 
     cat > config.toml <<EOF
-changelog-seen = 2
+change-id = 115898
 
 [rust]
 codegen-backends = []