[Master][Bump] Bump mono to match android. (#7685)

Commits are:
* [domain] Don't add NULL runtime to runtimes list (#18265) dc5902c243
* [eglib] Don't define _FORTIFY_SOURCE on Cygwin builds (#18312)  5f0704ce37
* Socket.BeginMConnect() should not attempt connections on unsupported address families. 99e3aed6ec
* [2019-10] [metadata] Size 0 Blob heap is ok when resolving assembly refs (#18338) 2faea92a92
* [FileSystemWatcher] Fix wrong behaviour when move multiple files out of watched folder on mac (#41963) df42020fe6

Complete diff is: bfcac9bd30...df42020fe6

mk/mono.mk

@@ -1,4 +1,4 @@
-NEEDED_MONO_VERSION := bfcac9bd309a05bcc1820e9ff1cb167708e4d6f4
+NEEDED_MONO_VERSION := df42020fe6f7a0a1b5fa59b481fb603d88d72348
 NEEDED_MONO_BRANCH := 2019-10
 
 MONO_DIRECTORY := mono