AUTOUIC: Fix merging of --blah options not being detected for Qt6

When merging a new set of uic options with a base set of options,
such as when a source file has a non-empty AUTOUIC_OPTIONS
property, the test for whether to support options starting with
two hyphens was only testing if the Qt major version was exactly
5 rather than at least 5. That had the effect of preventing such
options from being merged correctly when using Qt6, so new and
base options would both be present instead of the single merged
option from the source file properties. Update the version check
and rename function arguments to avoid misrepresenting what
they mean.

Note that uic accepts long option names with a single hyphen too.
See Qt docs for QCommandLineParser::ParseAsLongOptions
for confirmation.
46 jobs for !6588 with uic-options-merge-qt6 in 25 minutes and 4 seconds (queued for 1 second)
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