Make it possible to configure which repositories backports should be allowed from. This is similar to https://bugs.openjdk.org/browse/SKARA-1855, but for backports.

We want this to be able to configure that for example for openjdk/jdk21 backports should only be allowed from openjdk/jdk. In practice this means that the original commit being backported must be present in the configured repositories (openjdk/jdk in the previous example).