Restore _multibuild

- Update to 15.4:
* bsc#1214059, CVE-2023-39417: Disallow substituting a schema or
owner name into an extension script if the name contains a
quote, backslash, or dollar sign.
* bsc#1214061, CVE-2023-39418: Fix MERGE to enforce row security
policies properly.
* https://www.postgresql.org/docs/15/release-15-4.html

- Restore the independence of mini builds from the main build after
the -mini name change from April 4, 2023.
- Adjust icu handling to prepare for PostgreSQL 16.

- Overhaul postgresql-README.SUSE and move it from the binary
package to the noarch wrapper package.
- Change the unix domain socket location from /var/run to /run.