BUILD: makefile: bump the default minimum linux version to 4.17

BUILD: makefile: bump the default minimum linux version to 4.17

As explained during the 3.3-dev7 announcement below:
  https://www.mail-archive.com/haproxy@formilux.org/msg46073.html

no regularly maintained distro supports a kernel older than 4.18 anymore,
and KTLS is supported since 4.17. So it's about the right moment to bump
the default minimum kernel version supported by glibc and musl to
automatically cover new features. The linux-glibc-legacy target still
supports 2.6.28 and above.