ci: set MSRV to 1.69.0

Author: chriswk

.github/workflows/check.yaml

@@ -79,7 +79,7 @@ jobs:
     # https://docs.github.com/en/actions/learn-github-actions/contexts#context-availability
     strategy:
       matrix:
-        msrv: [1.56.1] # 2021 edition requires 1.56
+        msrv: [1.69.0] # 2021 edition requires 1.56
     name: ubuntu / ${{ matrix.msrv }}
     steps:
       - uses: actions/checkout@v3