ci: Use GITHUB_OUTPUT envvar instead of set-output command (#121)

`save-state` and `set-output` commands used in GitHub Actions are deprecated and [GitHub recommends using environment files](https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands/).

This PR updates the usage of `::set-output` to `"$GITHUB_OUTPUT"`

Instructions for envvar usage from GitHub docs:

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

.github/workflows/ci.yml

@@ -29,10 +29,10 @@ jobs:
         id: definitions
         run: |
           export MSRV=$(cat rust-toolchain | awk '{$1=$1};1')
-          echo "::set-output name=msrv::$MSRV"
+          echo "msrv=$MSRV" >> "$GITHUB_OUTPUT"
           export RAW_VERSIONS="stable beta $RUST_NIGHTLY_TOOLCHAIN $MSRV"
           export VERSIONS=$(echo $RAW_VERSIONS | jq -scR 'rtrimstr("\n")|split(" ")|.')
-          echo "::set-output name=versions::$VERSIONS"
+          echo "versions=$VERSIONS" >> "$GITHUB_OUTPUT"
 
   rustfmt:
     runs-on: ubuntu-latest