When RichFaces is released, it targets a recent Mojarra release, typically tracking what is available in JBoss EAP. This is reflected by the JSF version specified in the RichFaces BOM. (Note there is an issue RF-13017 with the jsf-api version in the released BOM).
RichFaces should work with Mojarra 2.1.23 in principle, unless one of the issues fixed in the recent Mojarra releases break something with RichFaces. If they do, please file an issue and we'll have that resolved in a subsequent RichFaces release.