Remove Qt5 support

[kelson42]

Fixes #1300

• Update README.md
• Remove ubuntu-22.04/qt5 (standart version) compilation from CI

[veloman-yunkan]

I think that the PR title is a little misleading (at least I hope so, since I want to be able to build kiwix-desktop on Ubuntu 22.04 jammy using Qt5).

[kelson42]

I think that the PR title is a little misleading (at least I hope so, since I want to be able to build kiwix-desktop on Ubuntu 22.04 jammy using Qt5).

I don't understand this comment. 2.6.0 will build only on Qt6 because there is no reason to keep supporting buggy old version of Qt.

[veloman-yunkan]

I meant that this PR doesn't enforce Qt6 and therefore building kiwix-desktop on Ubuntu 22.04 is still possible, and I like that because it enables me to keep developing also on my computer running jammy.

[kelson42]

@veloman-yunkan That is indeed correct, although bit surprising to me. Should we make a formal check in the .pro file?

[adamlamar]

LGTM. However note that the Ubuntu 22.04 OS is still supported for another year or so.

[kelson42]

I think that the PR title is a little misleading (at least I hope so, since I want to be able to build kiwix-desktop on Ubuntu 22.04 jammy using Qt5).

qmake nows exits with error if it does not have Qt5

[kelson42]

@legoktm Unfortunately, I'm in the impossibility tomget the package compiling properly with Qt6. Do you have maybe an idea why?