[LyX/master] Address Qt 6.7 deprecation warning

[Jean-Marc Lasgouttes]

Le 20/10/2024 à 08:04, Jürgen Spitzmüller a écrit :
> Am Samstag, dem 19.10.2024 um 19:24 +0200 schrieb Jean-Marc Lasgouttes:
>> It seems to me that this is usable from Qt 6.5:
>> https://www.qt.io/blog/unifying-qttimespec-within-qtimezone
>>
>> Or is it something else?
> 
> Yes, looks like it. The deprecation is only from 6.7 though.

So I can change the guard to check for Qt 6.5? It is better to have 
precise versions so that we know what #ifdefs can go when José forces us 
to require Qt 6.6 ;)

JMarc