Do not depend on changing toolchain version

If stable updates and introduces a new lint for example, that could
break our CI "randomly".
Thus, we do no longer depend on a mutable version of the toolchain, but
on a fixed one (that now has to be updated regularly, of course).

Signed-off-by: Matthias Beyer <[email protected]>

Author: matthiasbeyer

.github/workflows/msrv.yml

@@ -10,7 +10,7 @@ jobs:
       matrix:
         rust:
           - 1.70.0
-          - stable
+          - 1.73.0 # stable
           - beta
 
     steps:
@@ -36,7 +36,7 @@ jobs:
       matrix:
         rust:
           - 1.70.0
-          - stable
+          - 1.73.0 # stable
           - beta
     steps:
       - name: Checkout sources