Simplify spec file and replace hardcoded xrootd dependencies with a build conditional

[matyasselmeci]

This code builds just fine against XRootD 6 without any build script changes, and the RPM automatic dependency generator picks up the right version of xrootd to install due to the sonames.

Also, we can drop the BuildRequire on xrootd-server-libs, since it's brought in by xrootd-server-devel.

We do need to add globs to the .so files into the file list - they are suffixed with -5 or -6 depending on what version of xrootd they were built with.

[Copilot]

Pull request overview

This PR updates the RPM packaging for xrootd-s3-http to rely on RPM’s automatic dependency generation (via ELF NEEDED/sonames) instead of hard-coding XRootD major/minor version constraints, and adjusts the %files manifest to accommodate plugin .so filenames that include the XRootD plugin-version suffix (e.g., -5 / -6).

Changes:

• Drop explicit version-bounded BuildRequires/Requires on XRootD packages and rely on automatic dependency resolution.
• Remove redundant BuildRequires: xrootd-server-libs (covered by xrootd-server-devel).
• Replace explicitly versioned plugin filenames in %files with globs to match -5 / -6 suffixed modules and include additional plugin modules in the packaged file list.

[matyasselmeci]

This doesn't work for OSG on EL10: without the explicit version restriction, our Koji build for 25-main pulls in XRootD 6 from EPEL 10. Moving back to draft while I fix it...

[matyasselmeci]

OK, I added back the xrootd version dependencies, but now they're controllable with a build conditional: people using rpmbuild can add the flag --with xrootd6 to build with xrootd 6, or they can define the _with_xrootd6 1 macro in their build environments (as we do in some of the OSG build environments).