Stop using `libs-*` repo in favour of consistent use of Maven Central

[snicoll]

As we want to make sure we build and release against public artifacts available on Maven Central, let's stop using the libs-* repositories on repo.spring.io that are controlled by us and could potentially contain extra artifacts.

[snicoll]

The problem with this approach is that there are a few optional libraries that are not available on Maven central, so we need a repo for those in any case. One approach would be to create a repo that only contains those.

[snicoll]

The following dependencies non available on Maven Central are currently required by the build:

• com.ibm.websphere:uow:6.0.2.17
• io.spring.docresources:spring-doc-resources:0.1.0.RELEASE

For the doc resources, we had a similar problem in Spring Boot and decided to use a different mechanism to retrieve the doc resource (ping @bclozel). Looks like only uow should be made available.

[snicoll]

@bclozel confirmed spring-doc-resources can be downloaded differently so it looks like we're left with a single dependency to manage.

[snicoll]

We can use https://repo.spring.io/ext-release-local/ for optional artifacts that aren't on Maven Central.

[snicoll]

Blocked on #23282

[bclozel]

@snicoll this task is not blocked anymore from our asciidoc build's perspective.