Revert "Bump .NET SDK from 5.0 to 6.0"

This reverts commit 81c37b3 since CI
was picking up test failures when running the move-prover test suite:

```
test prover unit[simplify]::functional/unused_schema.move ... FAILED
Error: cannot extract version from `/opt/dotnet/tools/boogie`
```

Closes: aptos-labs#9995

scripts/dev_setup.sh

@@ -27,7 +27,7 @@ HELM_VERSION=3.2.4
 VAULT_VERSION=1.5.0
 Z3_VERSION=4.8.13
 CVC5_VERSION=0.0.3
-DOTNET_VERSION=6.0
+DOTNET_VERSION=5.0
 BOOGIE_VERSION=2.9.6
 PYRE_CHECK_VERSION=0.0.59
 NUMPY_VERSION=1.20.1