ifup doesn't complete interfaces configured with nmconnection key files on fedora

[yedayak]

Describe the bug

ifup in fedora doesn't complete since configured interfaces since the interface configuration is in /etc/NetworkManager/system-connections/ now by default, and _comp_compgen_configured_interfaces only looks for files like this: /etc/sysconfig/network-scripts/ifcfg-*.

To reproduce

1. Use a recent fedora system with configured interfaces in /etc/NetworkManager/system-connections/
2. type ifup <TAB>
3. No completions

Expected behavior

_comp_compgen_configured_interfaces should check for /etc/NetworkManager/system-connections/*.nmconnection files to get configured interfaces.

Versions (please complete the following information)

• Operating system name/distribution and version: fedora 39+
• bash version, echo "$BASH_VERSION": N/A
• bash-completion version, (IFS=.; echo "${BASH_COMPLETION_VERSINFO[*]}"): 2.15.0

Additional context

Found in #1280