Fix cargo upgrade to not break by using --incompatible

While `--incompatible` allows jumping to the next major version. it also means
it doing so breaks the build then the auto PR will break every week. Remove that
and we can see there are pending new major version numbers via `cargo outdated`.

default.nix

@@ -108,8 +108,7 @@ let
               echo ""
               echo "### cargo upgrade"
               printf "\n\`\`\`\n"
-              # --incompatible allows jumping to the next major version.
-              cargo upgrade --incompatible --manifest-path "$1/Cargo.toml" 2>&1
+              cargo upgrade --manifest-path "$1/Cargo.toml" 2>&1
               printf "\n\`\`\`\n"
 
               echo "### cargo update"