ci(pico-sdk): add version 2.1.1 to test matrix

Author: urish

.github/workflows/ci-pico-sdk.yml

@@ -9,7 +9,13 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        sdk_version: [1.2.0, 1.3.1, 1.4.0, 1.5.1, 2.0.0]
+        sdk_version:
+          - 1.2.0
+          - 1.3.1
+          - 1.4.0
+          - 1.5.1
+          - 2.0.0
+          - 2.1.1
     steps:
       - uses: actions/checkout@v4
       - name: Cache Pico SDK