Update repo install instruction for Fedora 45 and newer - #10638

Thanks for looking into Fedora 45+ DNF5 changes, Rostislav Krasny (@rostidev)!

I don't think we should instruct users to write manually to /usr/share/dnf5/repos.d/vscode.repo:

1. /usr/share/dnf5/repos.d is for distro/RPM-packaged repo files, not manual user configs: In Fedora 45+, default OS repository definitions (e.g., fedora.repo, fedora-updates.repo) were moved out of /etc into /usr/share/dnf5/repos.d to cleanly separate distribution-managed files from local administration.
2. /etc/yum.repos.d remains the standard path for manually added third-party repos: DNF5's reposdir configuration continues to include /etc/yum.repos.d (listed first, and where dnf5 config-manager addrepo writes by default), so the existing /etc/yum.repos.d/vscode.repo instruction continues to work on Fedora 45+ out of the box.
3. Immutable/atomic variants & FHS: Writing local configuration into /usr goes against Filesystem Hierarchy Standard conventions and will fail on atomic Fedora variants (such as Silverblue or Kinoite) where /usr is mounted read-only.

Because /etc/yum.repos.d/vscode.repo is still supported and recommended for manually added third-party repositories in Fedora 45+, keeping the single /etc/yum.repos.d/vscode.repo instruction avoids unnecessary branching and prevents users from writing into /usr.