Stable release 3.1.0

[cazfi]

The next major version of Freeciv, 3.1.0, has been released.

The major changes in Freeciv 3.1 are new Qt6- and gtk4-clients, improved balance for civ2civ3 ruleset, experimental meson based build system, and a whole lot of extra flexibility to create custom rulesets.
See the NEWS-3.1.0 since the last release line, 3.0.x.

For those who tried the betas and release candidate, there's a short list of changes since RC1 (translations).

Source code is available here. Compiled Windows packages for 3.1.0 aren't available yet, but should follow shortly; in the meantime, the 3.1.0-RC1 packages are still available and almost as good.

[cazfi]

Windows packages now available.