Bump rmeta version to fix rustc_serialize ICE

rust-lang#91407 changed the serialization format which leads to ICEs for nightly users such as rust-lang#91663 and linked issue.
Bumping the metadata version should lead to the cached files being discarded instead.

Author: the8472

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -48,7 +48,7 @@ crate fn rustc_version() -> String {
 /// Metadata encoding version.
 /// N.B., increment this if you change the format of metadata such that
 /// the rustc version can't be found to compare with `rustc_version()`.
-const METADATA_VERSION: u8 = 5;
+const METADATA_VERSION: u8 = 6;
 
 /// Metadata header which includes `METADATA_VERSION`.
 ///