Add support for building EL9 RPM

Further changes after the initial EL9 changes were needed to build the frontier-squid RPM on an EL9 machine. First, RPM signing has to be done as a separate step. Fortunately, this method is supported on EL7 and EL8, too. Second, there is no need to re-release all the OS-independent files when releasing the EL9 RPM, so these files are omitted for the EL9 release. The procedure will be to release the EL7/EL8 RPM, followed by the EL9 RPM.

The release scripts were tested to ensure they properly populate the repositories, with a test repo used for EL7 and EL8 and the official new EL9 repo used the EL9 test. The EL9 RPM that was released was tested on an EL9 machine to ensure it runs and supports Web Proxy Auto Discovery.