Bump required minimum version of libseccomp to 2.4.0

Major distributions already have libseccomp 2.5.x or newer.
Let's bump to the required minimum version to 2.4.0, which provides
SCMP_ACT_KILL_PROCESS, SCMP_ACT_LOG, SCMP_ARCH_PARISC, and
SCMP_ARCH_PARISC64.

Note, libseccomp 2.4.0 was released on 2019-03-15.

See also #38608.

src/shared/seccomp-util.h

@@ -153,9 +153,6 @@ _DEFINE_ABS_WRAPPER(SECCOMP_FATAL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL_RENAME(scmp_filter_ctx, sym_seccomp_release, seccomp_releasep, NULL);
 
 int parse_syscall_archs(char **l, Set **ret_archs);
-
-uint32_t scmp_act_kill_process(void);
-
 int parse_syscall_and_errno(const char *in, char **name, int *error);
 
 int seccomp_suppress_sync(void);