This allows sites using redirect=proxy to still generate ACME
certificates by not passing the acme requests through to the proxy.

We should also consider using this for other features available such as
own error documents, SSI and custom directives.

SSLProxyEngine is neither needed for http(s) redirects nor for non-SSL proxy configs.
Thus excluding it from these cases.

Based on a fix by Thom <thom@amsterdamtech.nl>

We sometimes had one and mostly had two capture groups. Now we always have two capture groups and thus can always use $2 for the redirect target.

Fixes #6144