Bump mono to get debugging fix when using the interpreter. (#15507)

New commits in mono/mono:

* mono/mono@2a19f878dab Backport dotnet/runtime#71436
* mono/mono@6051b710727 Bump nuget.exe to v6.2.1
* mono/mono@f354099a6b6 Set app context to null on thread detach
* mono/mono@6c7e72dd8e0 [mono] Fix a crash during stack trace construction if a this object is null.

Diff: https://github.com/mono/mono/compare/dffa5ab92245f2419238a35b7c2052e9a24036b4..2a19f878dab8d2e62123e0bf29453de553f5402a

mk/mono.mk

@@ -1,4 +1,4 @@
-NEEDED_MONO_VERSION := dffa5ab92245f2419238a35b7c2052e9a24036b4
+NEEDED_MONO_VERSION := 2a19f878dab8d2e62123e0bf29453de553f5402a
 NEEDED_MONO_BRANCH := 2020-02
 
 MONO_DIRECTORY := mono