MM-57245: Bump up Go version to 1.21 (#26512)

* MM-57245: Bump up Go version to 1.21

https://mattermost.atlassian.net/browse/MM-57245
```release-note
NONE
```

.github/workflows/codeql-analysis.yml

@@ -42,7 +42,7 @@ jobs:
     - name: Setup go
       uses: actions/setup-go@v2
       with:
-        go-version: '1.20'
+        go-version: '1.21'
       if: ${{ matrix.language == 'go' }}
 
 

.github/workflows/migration.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-22.04
     env:
       COMPOSE_PROJECT_NAME: ghactions
-      BUILD_IMAGE: mattermost/mattermost-build-server:20230904_golang-1.20.7
+      BUILD_IMAGE: mattermost/mattermost-build-server:1.21.8
       TEST_IMAGE: migration-test-image
     defaults:
       run:

.github/workflows/mmctl-test-template.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-22.04
     env:
       COMPOSE_PROJECT_NAME: ghactions
-      BUILD_IMAGE: mattermost/mattermost-build-server:20230904_golang-1.20.7
+      BUILD_IMAGE: mattermost/mattermost-build-server:1.21.8
     steps:
       - name: Checkout mattermost project
         uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1