ci: Use GITHUB_OUTPUT envvar instead of set-output command

`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/documentation.yml

@@ -30,7 +30,7 @@ jobs:
       # Cache dependencies
       - name: Get pip cache dir
         id: pip-cache
-        run: echo "::set-output name=dir::$(pip cache dir)"
+        run: echo "dir=$(pip cache dir)" >> "$GITHUB_OUTPUT"
 
       - name: Cache dependencies
         uses: actions/cache@v2

.github/workflows/tests.yml

@@ -28,7 +28,7 @@ jobs:
       # Cache dependencies
       - name: Get pip cache dir
         id: pip-cache
-        run: echo "::set-output name=dir::$(pip cache dir)"
+        run: echo "dir=$(pip cache dir)" >> "$GITHUB_OUTPUT"
 
       - name: Cache dependencies
         uses: actions/cache@v2