ruby: Upgrade to v3.1.4 (pt. 1, base image only) (forem#19871)

In forem#19776 we discovered that basing both the base container *and* the
app container off of `.ruby-version` causes a chicken-and-egg problem
wherein upgrading Ruby versions requires a few stop-and-gos. Let's make
those stops reviewable as separate chunks instead: use a new
`.ruby-version-next` for building base images, not the `.ruby-version`,
thus allowing GitHub Actions to fire off an automated build (which, when
complete, we can take the SHA sum from and use it to open a second PR
updating the app image to use it, and `.ruby-version` can be updated to
match `.ruby-version-next`).

.ruby-version-next

@@ -0,0 +1 @@
+3.1.4

scripts/build_base_ruby_image.sh

@@ -9,7 +9,7 @@ if [ "$(pwd)" != "$(git rev-parse --show-toplevel)" ]; then
 fi
 
 BUILD_PLATFORMS="${BUILD_PLATFORMS:-linux/amd64,linux/arm64}"
-RUBY_VERSION="${RUBY_VERSION:-$(cat .ruby-version)}"
+RUBY_VERSION="${RUBY_VERSION:-$(cat .ruby-version-next)}"
 IMAGE="ghcr.io/forem/ruby:${RUBY_VERSION}"
 
 if [ -z "${SKIP_PUSH:-}" ]; then