doc: update tracking issues of startup performance

The issues listed in the doc have been closed, and a new one is
currently being used to track snapshot integration.

PR-URL: https://github.com/nodejs/node/pull/40629


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>