Add UPDATING note about running make delete-old after libc++ 18 upgrade

PR: 279692
MFC after: 3 days

(cherry picked from commit 157802238b5aa7722aff40317fe6d05f5c975d71)
(cherry picked from commit 6b147f693b91f16db7ba6193611f6f4d2ed762a5)

Approved by: re (cperciva)