Allow case-insensitive [maxscale] section name

Up to maxscale 6.4 the [maxscale] header could be written in mixed case, e.g. [MaxScale] would also work. Starting with 22.08.0 this is no longer the case and only the lower case [maxscale].

While this case insensitive behavior was never really documented, and documentation examples seem to have used the lower case form exclusively as far as I can tell, we already had one customer running into problems on an upgrade to 22.08 due to this.

So we should either restore the previous behavior, or at least clearly document this as backwards incompatible change in the 22.08 release notes ...