set concurrency for the deploy job (#14448)

In the current GitHub Actions CI/CD setup, the `deploy` job is not
triggered when a PR is enqueued in the merge queue but only when it is
merged. Since concurrency is not configured for this job, deployments
may fail for later PRs if multiple PRs are merged in quick succession.
(e.g. the deployment for [this
commit](c418714518)
was successful, but the deployment for [this
commit](0a141ab7b8)
that was pushed to `main` immediately afterward failed. (edit: the
latter deployment seems to be rerun))

changelog: none

r? flip1995

.github/workflows/deploy.yml

@@ -8,6 +8,10 @@ on:
     tags:
       - rust-1.**
 
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: false
+
 env:
   TARGET_BRANCH: 'gh-pages'
   SHA: '${{ github.sha }}'

book/src/development/infrastructure/release.md

@@ -88,9 +88,6 @@ git push upstream stable
 After updating the `stable` branch, tag the HEAD commit and push it to the
 Clippy repo.
 
-> Note: Only push the tag once the Deploy GitHub action of the `beta` branch is
-> finished. Otherwise the deploy for the tag might fail.
-
 ```bash
 git tag rust-1.XX.0               # XX should be exchanged with the corresponding version
 git push upstream rust-1.XX.0     # `upstream` is the `rust-lang/rust-clippy` remote