Update upload-symbols with better logging and reading DEVELOPER_DIR to get Xcode path for CoreSymbolication (#16007)

Co-authored-by: Nick Cooke <36927374+ncooke3@users.noreply.github.com>

Crashlytics/CHANGELOG.md

@@ -1,3 +1,8 @@
+# Unreleased
+- [changed] Updated `upload-symbols` to version 3.21, updated error logging in
+  `upload-symbols` and started reading the `DEVELOPER_DIR` environment
+  variable for the path to Xcode for CoreSymbolication (#15896).
+
 # 12.11.0
 - [fixed] Fixed an issue where Crashlytics API calls were silently dropped if invoked immediately after Firebase initialization.