Everywhere: Require version >= 12 for GCC host compiler

So far we've gotten away with using GCC 11 for Lagom and to compile the
toolchain, but via #15795 we discovered a compiler bug that has been
fixed in the latest version but would error the build with CI's GCC 11.
Time for an upgrade :^)

We already use ubuntu-22.04 images in most places, so this is pretty
straightforward. The only exception is Idan's self-hosted runner, which
uses Ubuntu Focal. LibJS should build fine with GCC 11, still.

Toolchain/Dockerfile

@@ -8,8 +8,8 @@ RUN apt-get update -y \
         ccache \
         cmake \
         curl \
-        g++-11 \
-        gcc-11 \
+        g++-12 \
+        gcc-12 \
         e2fsprogs \
         genext2fs \
         git \
@@ -29,4 +29,4 @@ RUN apt-get update -y \
         tzdata \
         unzip \
     && rm -rf /var/lib/apt/lists/ \
-    && update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-11 900 --slave /usr/bin/g++ g++ /usr/bin/g++-11
+    && update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-12 900 --slave /usr/bin/g++ g++ /usr/bin/g++-12