build: relax qdarkstyle version bound

Historically, there have often been (visual) issues with new versions of qdarkstyle.
The upper bound restriction was mainly there for this reason: to ~force manually testing new versions.
There is no known issue with newer versions atm.

Remove the upper bound, as there have not been issues with newer versions recently,
and this makes it clear to e.g. packagers that it's fine to use newer versions.

Add a lower bound for a version that has been tested in the past and is known to work ok.

related https://github.com/spesmilo/electrum/issues/7361

contrib/requirements/requirements.txt

@@ -1,6 +1,6 @@
 qrcode
 protobuf>=3.12
-qdarkstyle<3.1
+qdarkstyle>=2.7
 aiorpcx>=0.18.7,<0.19
 aiohttp>=3.3.0,<4.0.0
 aiohttp_socks>=0.3