Bump jupyterhub to 1.4.2

[jgwerner]

• Updates jupyterhub/jupyterhub base images in Dockerfile to use v1.4.2 by default (version from JupyterHub release)
• Updates jupyterhub/k8s-hub base images in Dockerfile to use v1.1.1 by default (version from helm chart)
• Updates README example to build illumidesk/k8s-hub with latest version

Closes #600

[codecov[bot]]

Codecov Report

Merging #608 (0f73852) into main (7c4ffff) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #608   +/-   ##
=======================================
  Coverage   66.50%   66.50%           
=======================================
  Files          12       12           
  Lines         806      806           
=======================================
  Hits          536      536           
  Misses        270      270           

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