upgrade SAI xgs to 4.3.12.2 (#16354)

### Why I did it
Upgrade the SAI version to 4.3.12.2 to include the following changes:
- 4.3.10.2: [JIRA SONIC-68535][CSP CS00012273299] sai_query_attribute_capability for obj type SAI_OBJECT_TYPE_SWITCH
- 4.3.11.2: [JIRA SONIC-76883][CSP CS00012303308] SDK-367579: Skip flex port check when in iBodSync context
- 4.3.12.2: [JIRA SONIC-76472][CSP CS00012299690] Source suppressed broadcast packets count on interface TX discard statistics

##### Work item tracking
- Microsoft ADO **(number only)**: 24962692

#### How I did it
Upgrade the SAI version in sai.mk file.

#### How to verify it
Run SONiC basic test in SAI release pipeline: https://dev.azure.com/mssonic/internal/_build/results?buildId=361078&view=results

platform/broadcom/sai.mk

@@ -1,4 +1,4 @@
-LIBSAIBCM_XGS_VERSION = 4.3.8.1
+LIBSAIBCM_XGS_VERSION = 4.3.12.2
 LIBSAIBCM_BRANCH_NAME = REL_4.3_202012
 LIBSAIBCM_XGS_URL_PREFIX = "https://sonicstorage.blob.core.windows.net/public/sai/bcmsai/$(LIBSAIBCM_BRANCH_NAME)/$(LIBSAIBCM_XGS_VERSION)"