ci: update actions/download-artifact steps to v4

Update actions/download-artifact steps in code-coverage-deploy to v4.
This requires uploaded artifacts to use actions/upload-artifact v4;
therefore, this has also been updated. This fixes CI failures for
code-coverage-deploy.

Github deprecated actions/download-artifact@v2 [1], making it cause
automatic failures on workflows that use it. This causes our code
coverage deploy jobs to fail [2].

1: https://github.blog/changelog/2024-02-13-deprecation-notice-v1-and-v2-of-the-artifact-actions/
2: https://github.com/conjure-cp/conjure-oxide/actions/runs/11134871648/job/30943894187

.github/workflows/code-coverage-deploy.yml

@@ -234,7 +234,7 @@ jobs:
  })
  
  - name: Download the previous lcov.info (historical) file
- uses: actions/download-artifact@v2
+ uses: actions/download-artifact@v4
  with: |
  name: code-coverage-${{ env.num }}-${{ env.previous_artifact_id }}
  path: ./tools/code-coverage-diff/deploy-prev

.github/workflows/code-coverage.yml

@@ -73,7 +73,7 @@ jobs:
  echo "${{github.event.pull_request.number}}" > deploy/prnumber
 
  - name: Archive code coverage results for deployment
- uses: actions/upload-artifact@v3
+ uses: actions/upload-artifact@v4
  with:
  name: code-coverage-${{ steps.sha.outputs.sha }}
  path: deploy/**