build: bump Gentoo and Void versions in release-version

Move version bumping into scripts/bump-version.sh, rename the ebuild per release, reset the Void checksum, and add make pkg-checksums to fill it after the release.

scripts/update-checksums.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# Usage: scripts/update-checksums.sh
+# Fills the Void template checksum from the published release source tarball.
+# Run from the repo root after the release workflow has finished.
+set -eu
+
+T=pkg/void/srcpkgs/unitdore/template
+VER=$(sed -n 's/^version=//p' "$T")
+URL="https://git.warky.dev/wdevs/unitdore/releases/download/v$VER/unitdore-v$VER-source.tar.gz"
+
+SUM=$(curl -fsSL "$URL" | sha256sum | cut -d' ' -f1)
+sed -i "s/^checksum=.*/checksum=$SUM/" "$T"
+echo "void checksum for v$VER: $SUM"