strings, bytes: deprecate Title

Title doesn't handle Unicode punctuation and language-specific
capitalization rules. Replace the BUG comment with a Deprecated
one, suggesting a more robust alternative, and allowing Title
to be exposed to tooling.

Fixes #48367.

Change-Id: I952f1f37cd35b587a95893fb022827bdd9ec7de9
Reviewed-on: https://go-review.googlesource.com/c/go/+/359485


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Cherry Mui <cherryyz@google.com>