Revert "[Mono] Bump to match android on 66e2b840 (#9380)" (#9534)

This reverts commit c60759d857.

Android has issues with the bump due to the designer. At the moment we
are ahead, which is not a good situation. We rever to be in sync with
the other projects.

mk/mono.mk

@@ -1,4 +1,4 @@
-NEEDED_MONO_VERSION := 66e2b84002f037d7fd07d399f31196f05bd19597
+NEEDED_MONO_VERSION := 83105ba22461455f4343d6bb14976eba8b0b3f39
 NEEDED_MONO_BRANCH := 2020-02
 
 MONO_DIRECTORY := mono