Require Elixir 1.10, bump CI cache
We require Elixir 1.10 because it requires OTP 21+ which we need for the
:public_key.pkix_verify_hostname_match_fun function to securely download.
Also updating the cache key. When we need to bump again, simply change to v2, etc.
Co-authored-by: Wojtek Mach <wojtekmach@users.noreply.github.com>