scripts: Remove pre-3.10 kernel support

Reject unsupported kernel versions before download or patch generation.
Remove 2.x paths, RHEL 5/6 and Scientific Linux repositories, and
compatibility branches that cannot run with the Linux 3.10 minimum.

Keep the maintained ABT_KERNELS matrix unchanged.

scripts/generate-kernel-patch

@@ -236,6 +236,7 @@ fi
 
 # Strip patch level from the kernel version number.
 full_kver="$1"
+check_kernel_version "$full_kver" || exit $?
 kver="$(kernel_version "${full_kver}")"
 krel=${full_kver/^*}