[bugfix] reenable builds by switching to a different nexus

[line-o]

This is an attempt to re-enable builds of exist-db-6.x.x by switching to a different Nexus server.

https://repo.exist-db.org

The existing Nexus was kept in the parent-pom.

[reinhapa]

@line-o seems still some access remaining

Error:  Failed to execute goal on project exist-webdav: Could not collect dependencies for project org.exist-db:exist-webdav:jar:6.3.0-SNAPSHOT
Error:  Failed to read artifact descriptor for org.exist-db.thirdparty.com.ettrema:milton-api:jar:1.8.1.3
Error:  	Caused by: The following artifacts could not be resolved: com.ettrema:milton:pom:1.8.1.3 (absent): Could not transfer artifact com.ettrema:milton:pom:1.8.1.3 from/to evolvedbinary-releases (https://repo.evolvedbinary.com/repository/exist-db/): status code: 502, reason phrase: Bad Gateway (502)

[line-o]

Yep, they can be found here eXist-db/mvn-repo and I will seek to make them available in the new nexus.
Just to make that point clear they are on maven central but seem to have some kind of issue:
https://central.sonatype.com/search?q=com.ettrema

[line-o]

It is even weirder as the resource seems to be available in the new nexus
https://repo.exist-db.org/repository/exist-db/org/exist-db/thirdparty/com/ettrema/milton-api/1.8.1.3/milton-api-1.8.1.3.pom

[line-o]

I see the issue now: Milton is downloaded from our nexus and the parent project Milton-1.8.3 is missing there

[line-o]

I had contact with @ljo and we are on it.

[line-o]

I think we should revert this commit after it was merged to develop-6.x.x