Bump CI workflow and GoogleTest

[boeschf]

[TH] Github bumps its CI environment

• Upgrade actions to latest
  • cache
  • checkout
  • cmake
• Bump minimal tested versions of clang and GCC to 9
  • Maintenance: Bump versions across the board #1815 should do the rest, so we won't do it here.
• Remove Ubuntu 18.04 LTS and use 20.04 LTS a the new minimum.
  • GH drops support for 18.04

[FB] bump googletest version

• added version 1.12.1 as git submodule as requested in Formal dependency version management #1731
• previous version was ancient and caused heap corruptions at test cleanup stage
• updated deprecated gtest macros
• changed include paths in files that include gtest.h
• made stream operator in arbor/include/arbor/spike.hpp part of arb namespace - this fixes a compiler/linker bug for older compilers

[thorstenhater]

Only dubious part is CMake, but that might just be CMake, honestly.

[thorstenhater]

👍