Add Ubuntu 26.04 arm64 provisioning

This is a first tier-1 arm64 done with Packer. The initial Packer
scripts were taken from Ubuntu 26.04 x86_64 dir and modified for arm64.

Provisioning changes compared to arm64 24.04:
- Pulseaudio backport is removed:
    03-install-pulseaudio-16-bugfix-backports.sh
- apt: python3-pypdf2 -> python3-pypdf
- apt: libgstreamer-plugins-good1.0-dev -> -extra1.0-dev
- 04-clang.sh: llvm-toolchain-noble-20 -> -resolute-21
- 04-clang.sh: replace deprecated apt-key usage with a LLVM keyring and
  a signed APT source-list entry.
- 04-libclang.sh: pass -snld20 to 7-Zip for multi-hop symbolic links

Pick-to: 6.12
Task-number: QTQAINFRA-8002
Task-number: QTQAINFRA-5844
Change-Id: Ib4b00a837cba8a23a18ee70ece0ad4686f24fdba
Reviewed-by: Liang Qi <liang.qi@qt.io>