doc: remove 72-hour mentions in pull-requests.md

vsemozhetbyt noted that there were two mentions of the now-defunct
72-hour-rule in pull-requests.md. This change updates the text to
reflect current wait-time rules.

PR-URL: https://github.com/nodejs/node/pull/23309


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>