Use Arm-hosted runners (#581)

Documentation for these runners here https://gitlab.arm.com/tooling/gha-runner-docs
Note that only SLEEF upstream repo is allowed to use these runners.

Integrate runners via strategy.matrix.os, in order to facilitate customisation
of step (e.g. config) based on runner label.

Install OpenSSL since tester3 is enforced.
New runners do not seem to come with openssl or cmake pre-installed.

Use SVE enabled instances (c7g)
Use 2xlarge instances so tests run slightly faster without compromising on availability.

Remove aarch64 from cross build and tests (on Linux).
And update documentation/README.