Upgrade xgs SAI version to 8.4.31.0 (sonic-net#17059)

Why I did it
Upgrade the xgs SAI version to 8.4.31.0 to include the following changes:

8.4.22.0: [SDK upgrade][CSP CS00012314723][SAI_BRANCH rel_ocp_sai_8_4] SID:bcmtmPfcDdrScan thread takes 100% CPU utilization
8.4.23.0: [SDK upgrade][CSP CS00012290176[SAI_BRANCH rel_ocp_sai_8_4] SDK-323160: bcm_l3_ecmp_member_add returns Table Full error while ISSU
8.4.24.0:
[SDK upgrade]Merge "[CSP NA][SAI_BRANCH rel_ocp_sai_8_4] SID: Software LinkScan Not Catching Short Local/Remote Fault Events" into hsdk_6.5.27_SAI_8.4.0_GA
[SDK upgrade][CSP NA][SAI_BRANCH rel_ocp_sai_8_4] SID: Software LinkScan Not Catching Short Local/Remote Fault Events
8.4.25.0: [SAI_BRANCH rel_ocp_sai_8_4]CLONE - SAI - 8.4 - _brcm_sai_cosq_stat_get errors for CPU queue 41
8.4.26.0: [CSP CS00012307911] Fixed incorrect CPU related SAI port obj encoding/decoding in most subsystems
8.4.27.0: [CSP CS00012309154] [TD3] SAI_STATUS_INVALID_PARAMETER on setting SAI_BUFFER_POOL_ATTR_SIZE, OA crash
8.4.28.0: [CSP CS00012315552] Excessive logging from _brcm_sai_acl_tbl_grp_mbr_migration
8.4.29.0: [CSP CS00012321369] Fix TH2 regression with MMU/pool size
8.4.30.0: [SDK upgrade][CSP CS00012316299][SAI_BRANCH rel_ocp_sai_8_4] L3 entry delete failed when SER error is present
8.4.31.0: [CSP CS00012307911] Revert and limit scope of previous change due to WB issue.
Work item tracking
Microsoft ADO (number only): 26021230
How I did it
Upgrade the SAI version in sai.mk file.

How to verify it
Run advanced reboot on TH2 and TD3:

https://dev.azure.com/mssonic/internal/_build/results?buildId=422024&view=results
https://dev.azure.com/mssonic/internal/_build/results?buildId=423352&view=results
@saiarcot895 run warm reboot from 202012 to target image and they've passed
TH2: https://dev.azure.com/mssonic/internal/_build/results?buildId=423112&view=logs&j=76acabad-01e9-5c52-6fe6-d396d63e85d2&t=0d14fb40-14d5-50ca-4a23-af1778140cbf
TH: https://dev.azure.com/mssonic/internal/_build/results?buildId=423119&view=logs&j=76acabad-01e9-5c52-6fe6-d396d63e85d2&t=0d14fb40-14d5-50ca-4a23-af1778140cbf
TD3: https://dev.azure.com/mssonic/internal/_build/results?buildId=423074&view=logs&j=76acabad-01e9-5c52-6fe6-d396d63e85d2&t=0d14fb40-14d5-50ca-4a23-af1778140cbf

platform/broadcom/sai.mk

@@ -1,4 +1,4 @@
-LIBSAIBCM_XGS_VERSION = 8.4.21.0
+LIBSAIBCM_XGS_VERSION = 8.4.31.0
 LIBSAIBCM_DNX_VERSION = 7.1.111.1
 LIBSAIBCM_XGS_BRANCH_NAME = SAI_8.4.0_GA
 LIBSAIBCM_DNX_BRANCH_NAME = REL_7.0_SAI_1.11