Combine LTS and Latest versions categories in the component tree

As we decided to continue using categories in Installer 4.0, we should make the categories as intuitive to the user as possible. Current LTS and Latest releases are confusing mostly because of the dependencies, which result the LTS category contains some 5.9.x versions, for example, but not all of them.

LTS was added mainly to achieve an LTS target about one year ago, but it's not needed anymore. 

Let's have Latest releases + Preview and Archive. Latest releases will have some "ghost" nodes due to dependencies, but should not be as disturbing as with LTS category.

After LTS removed, we should indicate in Qt version node description, which version is an LTS version. Since 5.15 LTS we should also explain the difference between LTS version and LTS mode to explain open-source users, why they can still install some 5.15.x LTS patch releases.