CI/GitHub: Switch to GCC 8 (#25610)

* CI/GitHub: Switch to GCC 8

Build on GitHub Actions using GCC 8 as it's the oldest supported GCC

* Core/Misc: Attempt to fix GCC 8 build

(cherry picked from commit d7459ccced)

.github/workflows/gcc-build.yml

@@ -12,6 +12,7 @@ jobs:
     - name: Dependencies
       run: |
         sudo apt-get update && sudo apt-get install -yq libboost-all-dev
+        sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 100 --slave /usr/bin/g++ g++ /usr/bin/g++-8
     - name: Setup
       run: |
         mkdir bin