Maintainers.txt: Update source repository urls

Add 'mirror' to mirror/backup git repositories, and mark svn as
read-only and deprecated.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>

Author: jljusten2

Maintainers.txt

@@ -30,9 +30,9 @@ EDK II
 W: http://www.tianocore.org/edk2/
 L: https://lists.sourceforge.net/lists/listinfo/edk2-devel
 T: git - https://github.com/tianocore/edk2.git
-T: git - https://bitbucket.org/tianocore/edk2.git
-T: git - http://git.code.sf.net/p/tianocore/edk2
-T: svn - https://svn.code.sf.net/p/edk2/code/trunk/edk2
+T: git (mirror) - https://bitbucket.org/tianocore/edk2.git
+T: git (mirror) - http://git.code.sf.net/p/tianocore/edk2
+T: svn (read-only, deprecated) - https://svn.code.sf.net/p/edk2/code/trunk/edk2
 
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