Upgrade to Spring Boot 3.4.0, remove deprecated code and mark HtmxView as deprecated

[xhaggi]

Fixes gh-147 gh-148

[wimdeblauwe]

Thank you for your work @xhaggi. I find a bit a pity we closed @odrotbohm 's PR just like that. I think we could have merged it to honor his effort and then went from there. This PR is quite big, combining several things. I like to have smaller PR's. But it is ok for me now. Just 2 things:

1. I want to hold off this until we release 3.6.2 with the fix the exception handler methods
2. We should also update the README with the changes.

[odrotbohm]

Don't worry too much about my PR. All I wanted to convey was the extent of code we can get rid of by moving to the FragementRendering API, and it's that single class primarily. It's been the result of an early prototype I built to verify the APIs in Framework for suitability. Please feel completely free to move forward as you see fit.

[xhaggi]

Rebased. The last commit now contains an update to the README to point out the HTML fragment support.

[wimdeblauwe]

Looks good 👍