Consistently use repo.maven.apache.org to reference Maven Central

[wilkinsona]

With thanks the @breun for this comment on #33705:

According to https://issues.apache.org/jira/browse/MNG-5151 Maven first used repo1.maven.org, but later decided on making repo.maven.apache.org the default hostname for Maven Central, so maybe that should be used?

[wilkinsona]

According to this blog post, Sonatype consider both https://repo.maven.apache.org and https://repo1.maven.org to be canonical URLs for Maven Central. We currently use a mixture of the two. In the interests of consistency, I think there's still some value in making a change here.