QtDA test improvements: combine the install test cases

Combine the two almost identical test cases:
tst_ent_with_qtda_qt5

tst_ent_with_qtda_qt6

as "tst_ent_with_qtda"

Use the "ade_qtda_qt6" account for super LTS and all Qt6.x tests, and for rest (5.15) use the "ad_qtda_noqt6" account.

The if:

    if StrictVersion(product.based_on_qt) >= StrictVersion("6.2"):
        test.skip("Test not valid for versions higher than \
        6.2. Qt6Eligible required for installation.") 

can be removed, I don't see any use for that.

Also update TestRail plans.