bpf, docs: Rename legacy conformance group to packet

There could be other legacy conformance groups in the future,
so use a more descriptive name.  The status of the conformance
group in the IANA registry is what designates it as legacy,
not the name of the group.

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Link: https://lore.kernel.org/r/20240302012229.16452-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Author: Dave Thaler

Documentation/bpf/standardization/instruction-set.rst

@@ -127,7 +127,7 @@ This document defines the following conformance groups:
 * divmul32: includes 32-bit division, multiplication, and modulo instructions.
 * divmul64: includes divmul32, plus 64-bit division, multiplication,
   and modulo instructions.
-* legacy: deprecated packet access instructions.
+* packet: deprecated packet access instructions.
 
 Instruction encoding
 ====================
@@ -710,4 +710,4 @@ class of ``LD``, a size modifier of ``W``, ``H``, or ``B``, and a
 mode modifier of ``ABS`` or ``IND``.  The 'dst_reg' and 'offset' fields were
 set to zero, and 'src_reg' was set to zero for ``ABS``.  However, these
 instructions are deprecated and should no longer be used.  All legacy packet
-access instructions belong to the "legacy" conformance group.
+access instructions belong to the "packet" conformance group.