CI: build defmt feature only on stable rust

the MSRV of `defmt` does not guarantee that it builds on older rust
versions, accordingly we need to skip CI for older versions for feature
combinations where `defmt` is enabled.

Author: rursprung

.github/workflows/CI.yml

@@ -15,6 +15,11 @@ jobs:
       matrix:
         rust: [1.62.0, stable]
         features: ['use_alloc', 'use_alloc,defmt', 'use_heapless', 'use_heapless,defmt']
+        exclude:
+          - rust: 1.62.0
+            features: 'use_alloc,defmt'
+          - rust: 1.62.0
+            features: 'use_heapless,defmt'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4