add bootstrap config change info about rust.lld

the default value changes on `x86_64-unknown-linux-gnu`, and semantics kinda
as it will impact the target's default linker

src/bootstrap/src/utils/change_tracker.rs

@@ -175,4 +175,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Warning,
         summary: "The deprecated field `changelog-seen` has been removed. Using that field in `config.toml` from now on will result in breakage.",
     },
+    ChangeInfo {
+        change_id: 124129,
+        severity: ChangeSeverity::Warning,
+        summary: "`rust.lld` has a new default value of `true` on `x86_64-unknown-linux-gnu`. Starting at stage1, `rust-lld` will thus be this target's default linker. No config changes should be necessary.",
+    },
 ];