memfd-util: mention that F_SEAL_EXEC is supported since kernel v6.3

(cherry picked from commit f18365107051ce3b8c924f430a189ff027474e3b)

README

@@ -68,7 +68,7 @@ REQUIREMENTS:
            still function, but upstream support and testing are limited.
 
         Linux kernel ≥ 5.15 for MPOL_PREFERRED_MANY NUMA policy
-                     ≥ 6.3 for MFD_EXEC/MFD_NOEXEC_SEAL and tmpfs noswap option
+                     ≥ 6.3 for MFD_EXEC/MFD_NOEXEC_SEAL, F_SEAL_EXEC, and tmpfs noswap option
                      ≥ 6.5 for name_to_handle_at() AT_HANDLE_FID, SO_PEERPIDFD/SO_PASSPIDFD,
                                and MOVE_MOUNT_BENEATH
                      ≥ 6.6 for quota support on tmpfs

src/basic/memfd-util.c

@@ -96,7 +96,8 @@ int memfd_get_sealed(int fd) {
         if (r < 0)
                 return r;
 
-        /* We ignore F_SEAL_EXEC here to support older kernels. */
+        /* We ignore F_SEAL_EXEC here to support older kernels.
+         * It is supported since kernel v6.3 (6fd7353829cafc4067aad9eea0dc95da67e7df16). */
         return FLAGS_SET(seals, F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE);
 }