Look into recent HTMLTest issues

[mowies]

Recently, we had a few failing HTMLTest runs due to HTTP 429 errors (too many requests) from medium URL that were checked too often.

It seems that the HTMLTest cache is somehow broken since cache hits occur basically always in the pipelines, and external URLs should therefore only be checked every 2 weeks (source).

Example of a failing run due to HTTP 429 errors: https://github.com/keptn/lifecycle-toolkit/actions/runs/7725341537/job/21059354578