Fix inaccuracies in the Bootstrap 5 migration guide

[malberts]

Follows-up to #114, and is based on that branch so the corrections land with it.

Two claims in the new guide do not match the 6.0.0 source.

Carousel indicators. Only the children became buttons. Carousel.php renders div.carousel-indicators where it used to render ol.carousel-indicators, so the class survives and the element changes. A reader following the old row rewrites toward a selector that matches nothing. The extension's own stylesheet had to make this exact edit, .carousel ol.carousel-indicators to .carousel .carousel-indicators, so the affected selector shape is one that exists in the wild.

purgeParserCache. Run bare, the script exits with Must specify either --expiredate or --age. Passing --age 0 deletes every object created more than zero seconds ago, which reaches the whole parser cache because CacheTime caps each entry's expiry at $wgParserCacheExpireTime.

_{AI-authored — Claude Code, Fable 5 (max); two findings from a local review of #114, picked by @malberts, a third dropped at their call; diff not yet human-reviewed; each claim checked against the 6.0.0 source and MediaWiki 1.43 core, docs only so no tests.}