Enable incremental builds

[basil]

Like jenkinsci/plugin-pom#627 but for core and core components. The intended use case of this is to be able to more easily test PR builds of this repository in realistic use cases, albeit with the additional effort of adding a custom <repository>.

Testing done

Compared the differential between the partially flattened (new) https://repo.jenkins-ci.org/incrementals/org/jenkins-ci/jenkins/1.91-rc788.75b_1a_eca_b_8b_5/jenkins-1.91-rc788.75b_1a_eca_b_8b_5.pom and the unflattened (old) https://repo.jenkins-ci.org/public/org/jenkins-ci/jenkins/1.90/jenkins-1.90.pom and did not see anything of concern. I encourage reviewers to look at this differential for themselves. The only changes seem to be that properties seem to be in a random order (probably going through some HashMap internally somewhere) and the elements of the POM have been re-ordered according to the POM Code Convention (which we could re-order in the main branch if we wanted to reduce the diff even further).

Verified that I could successfully consume the resulting incremental build in Swarm Client by running mvn clean verify -DskipTests with

diff --git a/client/pom.xml b/client/pom.xml
index 00a3f34..999ec72 100644
--- a/client/pom.xml
+++ b/client/pom.xml
@@ -4,7 +4,7 @@
     <parent>
         <groupId>org.jenkins-ci</groupId>
         <artifactId>jenkins</artifactId>
-        <version>1.90</version>
+        <version>1.91-rc788.75b_1a_eca_b_8b_5</version>
         <relativePath />
     </parent>
 
@@ -88,6 +88,13 @@
             <id>repo.jenkins-ci.org</id>
             <url>https://repo.jenkins-ci.org/public/</url>
         </repository>
+        <repository>
+            <snapshots>
+                <enabled>false</enabled>
+            </snapshots>
+            <id>incrementals</id>
+            <url>https://repo.jenkins-ci.org/incrementals/</url>
+        </repository>
     </repositories>
 
     <pluginRepositories>