README: say kernel 4.15 is the minimum recommended

After various long discussions
(https://lists.freedesktop.org/archives/systemd-devel/2022-March/047587.html,
https://lwn.net/Articles/889610/), there is no clear answer what the minimum
version should be. Bumping the version above 3.15 doesn't allow us to make any
significant simplifications (unless we went *much* higher). In particular, even
renameat2() is not fully supported with latest kernel versions, e.g. nfs still
doesn't have it. And the bpf stuff is optional anyway. So let's just say that
4.15 is what we recommend, because it provides fairly complete cgroups-v2, but
without any removals of compat in the code.

README

@@ -30,14 +30,21 @@ LICENSE:
         LGPL-2.1-or-later for all code, exceptions noted in LICENSES/README.md
 
 REQUIREMENTS:
-        Linux kernel >= 3.15
-        Linux kernel >= 4.2 for unified cgroup hierarchy support
-        Linux kernel >= 4.10 for cgroup-bpf egress and ingress hooks
-        Linux kernel >= 4.15 for cgroup-bpf device hook
-        Linux kernel >= 4.17 for cgroup-bpf socket address hooks
-        Linux kernel >= 5.3 for bounded-loops in BPF program
-        Linux kernel >= 5.4 for signed Verity images support
-        Linux kernel >= 5.7 for BPF links and the BPF LSM hook
+        Linux kernel ≥ 3.15
+                     ≥ 4.5 for pids controller in cgroup v2
+                     ≥ 4.6 for cgroup namespaces
+                     ≥ 4.9 for RENAME_NOREPLACE support in vfat
+                     ≥ 4.10 for cgroup-bpf egress and ingress hooks
+                     ≥ 4.15 for cgroup-bpf device hook and cpu controller in cgroup v2
+                     ≥ 4.17 for cgroup-bpf socket address hooks
+                     ≥ 5.3 for bounded loops in BPF program
+                     ≥ 5.4 for signed Verity images
+                     ≥ 5.7 for BPF links and the BPF LSM hook
+
+        Kernel versions below 4.15 have significant gaps in functionality and
+        are not recommended for use with this version of systemd. Taint flag
+        'old-kernel' will be set. Systemd will most likely still function, but
+        upstream support and testing are limited.
 
         Kernel Config Options:
           CONFIG_DEVTMPFS

src/basic/def.h

@@ -74,4 +74,4 @@
 /* Path where systemd-oomd listens for varlink connections from user managers to report changes in ManagedOOM settings. */
 #define VARLINK_ADDR_PATH_MANAGED_OOM_USER "/run/systemd/oom/io.system.ManagedOOM"
 
-#define KERNEL_BASELINE_VERSION "3.15"
+#define KERNEL_BASELINE_VERSION "4.15"