Add erc--skip message property

Add erc--skip message property

* lisp/erc/erc-backend.el (erc-server-connect): Add `erc--skip'
property to `erc--msg-prop-overrides' so that timestamps only show up
with the first server-sent message.
(erc-server-PRIVMSG): Move `erc--msg-prop-overrides' declaration to
top-level.
* lisp/erc/erc-button.el (erc-button--display-error-notice-with-keys):
Use `erc--skip' msg prop instead of `erc-stamp--skip' variable.
* lisp/erc/erc-stamp.el (erc-stamp--skip): Remove variable.
(erc-stamp--allow-unmanaged, erc-stamp--allow-unmanaged-p): Rename
former to latter to remain consistent with convention used by other
quasi-internal compatibility-related switches.
(erc-add-timestamp): Check `erc--skip' property instead of deleted
variable `erc-stamp--skip'.
* lisp/erc/erc.el (erc--msg-props): Mention `erc--skip' in doc.
(erc--check-msg-prop): Doc.
(erc--memq-msg-prop): New function.
(erc--ranked-properties): Add `erc--skip'.
* test/lisp/erc/erc-scenarios-stamp.el
(erc-scenarios-stamp--legacy-date-stamps): Revise to expect
"opening connection.." to appear above first stamp.
* test/lisp/erc/erc-tests.el (erc--memq-msg-prop): New test.
(Bug#60936)

(cherry picked from commit 6000e48e0d7d5742ba817942f1b0dbbda9315ddc)