properly drop support for nedb

[evilham]

As documented in: #1858 (review) and matrix-org/matrix-appservice-bridge#521

In node 22, some deprecated APIs were removed that break nedb.

Anyway, long story short: v4.0.0 of matrix-appservice-irc cannot currently be used if using nedb as a database; this ought to be documented along with a migration path.
At the very least the sample config file shouldn't include mentions of nedb, if anything only to mention that v3.0.5 was the last one to support it:

matrix-appservice-irc/config.sample.yaml

Line 698 in c261a5e

# database engine (must be 'postgres' or 'nedb'). Default: nedb

UPDATE: realised that it seemed that the consensus is to drop support for nedb, changed the title to better represent that instead of pointing in the opposite direction