Support for newer Bootstrap versions

[pv42]

Bootstrap 5.3.0 supports color themes (e.g. dark mode), which i would like to use.

Just chanaging the version seems straigtforeward since there seem to be no breaking changes on the code side, but to preserve the access to the old version via the previous util::include APIs I suggest adding new includes for newer versions.
The simplest would be adding include_cdn_53() include_cdn_js_53() and include_inline_53().

But adding a Bootstrap version enum as argument to the include version would probably be a better solution.

[isosphere]

Theming is a valuable feature. If there are truly no breaking changes I would be in favour of simply migrating to 5.3.0 without backwards compatibility - it'll create less technical debt long-term; unless there is a good reason I am not considering to support multiple versions.

If bootstrap is faithfully using semantic versioning, it should be very painless, we'll just have more features on our todo list.

We'd want to review https://getbootstrap.com/docs/5.2/migration/ and https://getbootstrap.com/docs/5.3/migration/ before proceeding.

[pv42]

There seem to be some changes to the style itself, for me the background of an active accordion header looks slightly darker for example but i can't quite find that mentioned in the change notes.