[202012] BRCM SAI 4.3.5.1-8 Pick up fix for PFCWD getting continuously triggered/restored when pause frames are sent continuously to both queues of a port

[gechiang]

This is to pick up BRCM SAI 4.3.5.1-8 fix which contains the following fix:

1. CS00012211718 [4.3] Pfcwd getting continuously triggered/restored when pause frames are sent continuously to both queues of a port (TD2/Th/Th2/TD3) MSFT Default

Preliminary tests look fine. BGP neighbors were all up with proper routes programmed
interfaces are all up
Manually ran the following test cases on 7050CX3 (TD3) T0 DUT and all passed:

     fib/test_fib.py
     vxlan/test_vxlan_decap.py
     fdb/test_fdb.py
     decap/test_decap.py
     ipfwd/test_dip_sip.py 
     ipfwd/test_dir_bcast.py
     acl/test_acl.py
     vlan/test_vlan.py
     platform_tests/test_reboot.py

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106

Description for the changelog

[202012] BRCM SAI 4.5.3.1-8 Picked up SAI fix for CS00012211718.

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