Mer pluginspec: use QTCREATOR_COMPAT_VERSION

See cc7c3c9 (Add QTCREATOR_COMPAT_VERSION)

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
Reintroduce "Jolla: Mark Android and QNX plugins as experimental."

This reintroduces commit 79b9b78528d669b18620095bdc40a7195d295670,
temporarily reverted prior to mer-3.1 merge with commit
e6dfc4101b783ec914df7d9281d10b5798bb6b5f.

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>