Mirror manifest contains non-public repository

[Al Sutton]

https://android.googlesource.com/platform/toolchain/jdk/jdk21/ isn't public (yet), but a reference to it was introduced in c31d267750898e25494c0203f40e1e2bc700229b - mirror/manifest - Git at Google (googlesource.com) which means the `repo sync` on an AOSP mirror will now fail to complete cleanly.