3.0.6 (& 2.6.8?)

 It's been a bit of a "you have to run to stay in the same place" time
since 3.0.5 release.

- Latest libcurl release complains about freeciv's usage of long-deprecated
API
- gcc-13 (to be officially released April 2023) gave one new error by
default when building freeciv
- When gcc-13 used in a mode enabling draft of the new C23 standard (or
C2x, which is expected to get realized as C23), there's a couple of build
failures
- g++-13 c++23 standard mode (affecting Qt-client and ruledit build) is
still to be tested.

 I'd like a 3.0.6 release addressing these out relatively soon, especially
for bleeding edge linux distributions to take in. Also, by the policy of
still keeping S2_6 maintained enough of it to build on newer systems, I
think it would be good to release also 2.6.8 with these issues fixed.


 - ML