Easy first-time contributor issue for Training

[stevepiercy]

Hi Everyone,

Here is an easy, first-time contributor issue:

Replace links to `https://training.plone.org/5/*` with `https://training.plone.org/*` across Plone org

There are 81 instances of https://training.plone.org/5/* across all the Plone organization's GitHub repositories that need to be updated. Many hands make light work.

Please take one and ping me to review your PR. Thank you!

[stevepiercy]

Thanks to everyone who has contributed, we are now down to 36 files that contain the soon to be deprecated URL https://training.plone.org/5/.

Enable the new GitHub code search to use exact matching.

Then search for the files.