prepare for Qt 6.8 deprecations (#1346)

prepare for Qt 6.8 deprecations (#1346)

* fix Qt 6.8.0 deprecation warnings.

Also, the exif writer now prefers to establish a timestamp from
GPS_IFD_TAG_DATESTAMP/GPS_IFD_TAG_TIMESTAMP if they exist, as the
exif reader does.

* avoid installing *.debug_information modules

which appeared with 6.7.0

* centralize decision on lightweight time zones.

* correct sort order for includes, duplicates

* remove debug statement

* remove const from declaration.

* simplify lightweight timezone usage via

qt version depependent defined constants.

* fix for QTimeZone not a QLiteralType

* cleanup lightweight time constants.

* enhance exif offset test

* add missing reference

* fix test comment

* revert changes to exif writer regarding gps tags.

I beleive the original intent of the writer is to create the gps tags
from a waypoint.

* update qt modules

* qt 6.8 work

1. allow builds to succeed without webengine by default.  This is
needed by CI until aqt catches up.
2. macos minos change for 6.8
3. add macos 6.8.0 to CI, remove macos-12 build.

* avoid Wunused-const-variable

* add 6.8.0 to ci for windows