<utility> Deprecate std::rel_ops & resolve #403

[SuperWig]

Description

Just another check box to tick for #64

Perhaps "the std::rel_ops operators are deprecated..." is better?

Checklist

Be sure you've read README.md and understand the scope of this repo.

If you're unsure about a box, leave it unchecked. A maintainer will help you.

• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before automated testing is enabled on GitHub,
  leave this unchecked for initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository,
  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
  that's already listed in NOTICE.txt, that's fine, but please mention it.
  If they were derived from any other project (including Boost and libc++,
  which are not yet listed in NOTICE.txt), you must mention it here,
  so we can determine whether the license is compatible and what else needs
  to be done.

[AdamBucior]

I think you should also deprecate namespace, so if someone uses using namespace std::rel_ops; he receives the warning.

[SuperWig]

@AdamBucior I didn't realise that was a thing. 👍

[CaseyCarter]

Just a few mechanical changes, and I'm sure we'll all want to bicker about the text of the deprecation message.

[BillyONeal]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[CaseyCarter]

LGTM.

[StephanTLavavej]

Looks great, thank you!

[BillyONeal]

Thanks for your contribution!