Generate bundles concurrently in CI

[geriux]

Related PRs:

• [Mobile] - Remove unused pre/post bundle commands WordPress/gutenberg#58305

This PR aims to seep up some CI checks.

• Adds the concurrently library to run the bundle generation commands concurrently to speed up the Build JS Bundle step.
• Removes depends_on for the Build JS Bundle step to speed up CI. Currently, we lack of node_module cache, etc.

To test CI checks should pass.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.