CI: Rotate expired Docker Hub secrets

[benhayes21]

Problem

The DOCKERHUB_USERNAME and DOCKERHUB_TOKEN secrets in GitHub Actions are approximately 4 years old and the token has expired, causing the Linux CI job to fail with:

Error response from daemon: Get "https://registry-1.docker.io/v2/": unauthorized: incorrect username or password

This is blocking PR #409 from fully passing.

Steps to fix

1. Log into the Docker Hub account that owns coreoasis/ktools-builder
2. Go to Account Settings → Security → Access Tokens and generate a new token
3. Update the following secrets in github.com/OasisLMF/ktools/settings/secrets/actions:
   • DOCKERHUB_USERNAME — Docker Hub account name (not email)
   • DOCKERHUB_TOKEN — the newly generated token
4. Re-run the failed Linux job on PR Fix CI: update macOS runner to macos-14 (Apple Silicon) #409