chore(deps): update ghcr.io/ublue-os/silverblue-main:latest docker digest to a21378c - #4448

Starting April 2026, the CodeQL Action will skip computing file coverage information on pull requests to improve analysis performance. File coverage information will still be computed on non-PR analyses. To opt out of this change, create a custom repository property with the name `github-codeql-file-coverage-on-prs` and the type "True/false", then set this property to `true` in the repository's settings.