Upgrade GitHub action upload-artifact

Update `actions/upload-artifact` from `v2` to `v4` due to [deprecation notice](https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/) (scheduled for June 30, 2024).

https://github.com/FuelLabs/fuel-explorer/blob/main/.github/workflows/pr-tests.yml#L51
https://github.com/FuelLabs/fuel-explorer/blob/main/.github/workflows/pr-tests.yml#L95

Open .github/workflows/pr-tests.yml and inspect the upload-artifact references at lines 51 and 95. Update both references from v2 to v4, then verify that the workflow still parses and both artifact-upload steps use the requested version.

Written by the indexing model from the issue text.