Upgrade xgs SAI version to 10.1.21.0 (sonic-net#19334)

Why I did it
Upgrade the xgs SAI version to 10.1.21.0 to include the following changes:

10.1.8.0: Support _brcm_sai_port_unlos_set when external port_phy_addr is 0xff
10.1.9.0: [CS00012330893]With PFC test, egress drop observed for lossless queues
10.1.11.0: Include the bcmsim in the odp package
10.1.13.0: handle bind of egress and ingress groups to same lag
10.1.14.0: Backport SDK-381920 to rel_ocp_sai_10_1 High CPU due to SDK calling soc_dnxc_port_resource_get
10.1.16.0: [CSP CS00012336644] callback called twice with same event type
10.1.17.0: [CSP 00012320736][SAI_BRANCH rel_ocp_sai_10_1]Back port MMU related Fixes to 10.1 specific to LTSW/TH4 devices-HSDK
10.1.18.0: [CSP CS00012351984] Backport SONIC-81526 to 10.1 Fixes for rate class bound setting and UCQ stats
10.1.19.0: Reset EGR_L3_NEXT_HOP_2 for TH3
10.1.20.0: Sonic crash due to unsupported return from SAI_SWITCH_ATTR_ECMP_DEFAULT_HASH_OFFSET
Work item tracking
Microsoft ADO (number only): 28356083
How I did it
Upgrade the SAI version in the sai.mk file.

How to verify it
Run the SONiC basic test:
https://elastictest.org/scheduler/testplan/66742d7ef6db3102cef61056

platform/broadcom/sai.mk

@@ -1,4 +1,4 @@
-LIBSAIBCM_XGS_VERSION = 10.1.7.0
+LIBSAIBCM_XGS_VERSION = 10.1.21.0
 LIBSAIBCM_DNX_VERSION = 10.1.25.0
 LIBSAIBCM_XGS_BRANCH_NAME = SAI_10.1.0_GA
 LIBSAIBCM_DNX_BRANCH_NAME = SAI_10.1.0_GA