Replace cache-clearing service in suggested purging process
Replace cache-clearing service in suggested purging process

This updates the guidance on purging a page from the cache to be less
focused on the cache-clearing service and to reflect current GOV.UK
reality (where most pages are cached for 5 mins)

It replaces the instructions on how to clear the cache to use the
Jenkins task that is provided for this (which was updated in
https://github.com/alphagov/govuk-puppet/pull/11663 for this purpose)

This has been done in order to reduce dependencies on the cache-clearing
service to prepare for it's potential retirement.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs