Support building site with Ruby 3.x

[kwin]

Currently the Ruby version set in https://github.com/Adobe-Consulting-Services/adobe-consulting-services.github.io/blob/master/.ruby-version is 2.6.6.

I failed to compile 2.6.6 for my M2 Mac due to the following issues:

1. error: implicit declaration of function 'ffi_prep_closure' on M1 macOS Big Sur ffi/ffi#869 and
2. Can't install ruby on M1 Mac "use of undeclared identifier 'RSA_SSLV23_PADDING'" postmodern/ruby-install#409

Although there may be workaround like postmodern/ruby-install#409 (comment) for installing 2.6.6 it should be possible to built the site with a modern Ruby 3.x version (which currently fails due to outdated dependencies in https://github.com/Adobe-Consulting-Services/adobe-consulting-services.github.io/blob/master/Gemfile.lock.