[202012][Mellanox] Change MFT version to 4.21.0-100 (#13956)

- Why I did it
Update MFT version to 4.21.0-100 to include a fix for an issue reported using mlxlink on qsfp-dd

- How I did it
Update mft.mk

- How to verify it
Run regression on Mellanox platforms

Author: dgsudharsan

platform/mellanox/mft.mk

@@ -1,7 +1,7 @@
 # Mellanox SAI
 
-MFT_VERSION = 4.18.0
-MFT_REVISION = 106
+MFT_VERSION = 4.21.0
+MFT_REVISION = 100
 
 export MFT_VERSION MFT_REVISION