Support Fedora 41 in Dockerfile

This is needed for "make check" to work on a Fedora 41 host where the
FROM directive is overridden to fedora:41.

- Drop the workaround for gdb-headless, the old version no longer exists
  in F41 and the bug has been fixed in both F40 and F41:
  https://bugzilla.redhat.com/show_bug.cgi?id=2310156

- Don't try to remove rpm-sign-libs and python3-rpm on F41 since dnf5 no
  longer pulls them in

tests/Dockerfile.fedora

@@ -46,7 +46,7 @@ RUN dnf -y install \
   tar unzip gzip bzip2 cpio xz p7zip \
   pkgconfig \
   /usr/bin/systemd-sysusers \
-  "gdb-headless < 15" \
+  gdb-headless \
   dwz \
   fsverity-utils fsverity-utils-devel \
   pandoc \
@@ -65,8 +65,7 @@ RUN rpm -e --nodeps --nodb \
 	rpm \
 	rpm-libs \
 	rpm-build-libs \
-	rpm-sign-libs \
-	python3-rpm
+	$(which dnf5 >/dev/null 2>&1 || echo rpm-sign-libs python3-rpm)
 
 WORKDIR /
 CMD /bin/bash