nd6: Remove DRAFT_IETF_6MAN_IPV6ONLY_FLAG and EXPERIMENTAL options

The draft-ietf-6man-ipv6only-flag has been obsoleted by RFC 8925.
Remove the EXPERIMENTAL compile option from the kernel and remove
DRAFT_IETF_6MAN_IPV6ONLY_FLAG from userland.
This compile option was not enabled by default.
Also regenerate src.conf.5.

Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D56228

share/man/man5/src.conf.5

@@ -1,5 +1,5 @@
 .\" DO NOT EDIT-- this file is @generated by tools/build/options/makeman.
-.Dd April 3, 2026
+.Dd April 10, 2026
 .Dt SRC.CONF 5
 .Os
 .Sh NAME
@@ -721,8 +721,6 @@ An alternate bootstrap tool chain must be provided.
 .It Va WITHOUT_EXAMPLES
 Avoid installing examples to
 .Pa /usr/share/examples/ .
-.It Va WITH_EXPERIMENTAL
-Include experimental features in the build.
 .It Va WITHOUT_FDT
 Do not build Flattened Device Tree support as part of the base system.
 This includes the device tree compiler (dtc) and libfdt support library.