[DOCS] Rename LLVM Security Group to LLVM Security Response Group.

Rename LLVM Security Group to LLVM Security Response Group. Take
the opportunity to canonicalise security group and Security Group
to LLVM Security Response Group.

At the 2024-11-19 LLVM Security Group meeting [1] we discussed that in
practice the LLVM Security Group was performing an incident response
role, but it was not proactively adding additional testing, fuzzing
and hardening. We do not want projects that use LLVM to see the
LLVM Security Group as guaranteeing security for LLVM.

We decided that it would be useful to rename the group to
LLVM Security Response Group as that reflects the work that it is
doing.

There may be a case for a proactive security group with a different
remit, but this is out of scope of this commit.

[1]
https://discourse.llvm.org/t/llvm-security-group-public-sync-ups/62735/32

Author: smithp35