Drop bpftool version requirement to 4.18.0
    
    The CentOS Stream 8 bpftool package has the skeleton and object
    generation features introduced in bpftool 5.6.0 and 5.13.0
    respectively backported to 4.18.0. The meson script doesn't know
    this, so we downgrade the required bpftool version to 4.18.0 to
    allow builds to pass on CentOS Stream 8.
    
    (cherry picked from commit 8d1b5fe8ee53ffd5885bee6131606d79dbbdde91)