upgrade xgs SAI version to 8.4.21.0 (sonic-net#16805)

Upgrade the xgs SAI version to 8.4.21.0 to include the following changes:

8.4.21.0: [CSP CS00012316669][SAI_BRANCH rel_ocp_sai_8_4] FP destroy API behavior change to avoid traffic leaks
8.4.20.0: [CSP CS00012312900] Max path used as 0 in ordered ECMP replace.
8.4.19.0: [CSP CS00012301679] sai_query_attribute_capability SAI_OBJECT_TYPE_SWITCH, fix few attrs in previous checkin
8.4.18.0: [CSP CS00012310706] Add SAI_TUNNEL_SUPPORT to azure pipeline build files
8.4.16.0: [CSP CS00012301679] sai_query_attribute_capability for obj type SAI_OBJECT_TYPE_SWITCH
8.4.15.0: [SAI_BRANCH rel_ocp_sai_8_4] Port SONIC-75025 to SAI 8.4
8.4.14.0: [CSP CS00012306356] Change log level of sai_bulk_object_get_stats, unsupported object type to warning
8.4.13.0: [CSP CS00012302193] backport SONIC-72912 jira on SAI 8.4 branch
8.4.12.0: [CSP CS00012296541][SAI_BRANCH rel_ocp_sai_8_4] Preformance improvement for ECMP from SDK-354625
8.4.11.0: [CSP CS00012293985] Port SONIC-74816 fix to 8.4.
8.4.10.0: [CSP NA/SID-26013][SAI_BRANCH rel_ocp_sai_8_4] SID - L3 multicast packet drop due to wrong VFI derivation - SDK-350470
8.4.9.0: [CSP NA/SID-25917][SAI_BRANCH rel_ocp_sai_8_4] SID-Crash in ALPM algorithm during entry split SDK-343694
8.4.8.0: [CSP CS00012275265][SAI_BRANCH rel_ocp_sai_8_4] SID Deadlock in linkscan callback during flexport operations
8.4.7.0: [CSP CS00012284142] Fixed MMU buffer config issue with multicast queues
8.4.6.0: [CSP CS00012275454] sai_object_type_get_availability failed with SAI_STATUS_INVALID_PARAMETER; [CSP CS00012284121] [SAI_BRANCH rel_ocp_sai_8_4] SID - L2_ENTRY Table Lookups May Miss
8.4.4.0: [CSP CS00012287462] Uplift tunnel fix from SONIC-73462
8.4.2.0: Fixing the issue with SAI_QUEUE_STAT_DROPPED_PACKETS retrieval; Enable/Disable bitmask for egress stats; SAI - OCP SAI 8.4 - SAI: Reduce Index data type union _brcm_sai_indexed_data_t size to be below 2k.; Cut Down Version - Port Tpid Compilation Issue Fix

Signed-off-by: zitingguo-ms <zitingguo@microsoft.com>

platform/broadcom/sai.mk

@@ -1,4 +1,4 @@
-LIBSAIBCM_XGS_VERSION = 8.4.0.2
+LIBSAIBCM_XGS_VERSION = 8.4.21.0
 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