Remove `readonly` from GCHandle field on Gen2GcCallback (#26509)
Remove `readonly` from GCHandle field on Gen2GcCallback (#26509)

GCHandle is mutable, and this type does call Free on the handle, but with it marked as readonly, the mutations performed by Free will be done on a copy rather than on the original.

Signed-off-by: dotnet-bot <03c4cb6107ae99956a7681f2c97ff7104809a2bd@microsoft.com>

shimingsg/corefx,ViktorHofer/corefx,wtgodbe/corefx,shimingsg/corefx,shimingsg/corefx,ericstj/corefx,ViktorHofer/corefx,ViktorHofer/corefx,shimingsg/corefx,wtgodbe/corefx,ViktorHofer/corefx,ericstj/corefx,wtgodbe/corefx,wtgodbe/corefx,wtgodbe/corefx,ViktorHofer/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,ericstj/corefx,shimingsg/corefx,ericstj/corefx,wtgodbe/corefx,ViktorHofer/corefx,wtgodbe/corefx,shimingsg/corefx,ericstj/corefx,ViktorHofer/corefx