Fix local variable persistence in erc-stamp

Fix local variable persistence in erc-stamp

* etc/ERC-NEWS: Mention renaming of `erc-munge-invisible-property'.
* lisp/erc/erc-stamp.el (erc-stamp-mode, erc-stamp-disable): Remove
correct function from `erc-mode-hook'.
(erc-stamp--recover-on-reconnect): Revise doc string.
(erc-munge-invisibility-spec, erc-stamp--manage-local-options-state):
Mark former name as obsolete and rename to latter.  Don't use helper
macro meant only for local modules.  This bug originated from c68dc778
"Manage some text props for ERC insertion-hook members", which stemmed
from bug#60936.
(erc-stamp--setup, erc-hide-timestamps, erc-show-timestamps)
(erc-toggle-timestamps): Use new name for
`erc-munge-invisibility-spec'.
* lisp/erc/erc.el (erc--restore-initialize-priors): Raise error at
runtime if mode var doesn't belong to a local module.
* test/lisp/erc/erc-stamp-tests.el (erc-stamp-tests--insert-right)
(erc-timestamp-intangible--left): Use new name for
`erc-munge-invisibility-spec'.
* test/lisp/erc/erc-tests.el (erc--refresh-prompt): Shadow
`erc-last-input-time'.
(erc--restore-initialize-priors): Add error form to expected
expansion, and skip test on Emacs 27.
* test/lisp/erc/resources/erc-scenarios-common.el
(erc-scenarios-common--make-bindings): Shadow `erc-last-input-time'.