Support Ruby 3.0.3

[jrgriffiniii]

Derived from samvera/maintenance#77

[jrgriffiniii]

#135 (comment) must be addressed before this can be completed.

[jrgriffiniii]

Ldp::Client#head is somehow creating resources against the rdf-ldp server, but I am otherwise progressing with this upgrade.

[jrgriffiniii]

Unfortunately, supporting Rails 6.0.z releases may well require that support for Ruby 2.5.z is dropped.

Bundler could not find compatible versions for gem "nokogiri":
  In Gemfile:
    rails (~> 6.0.0) was resolved to 6.0.4.7, which depends on
      actiontext (= 6.0.4.7) was resolved to 6.0.4.7, which depends on
        nokogiri (>= 1.8.5)

    ldp was resolved to 1.0.3, which depends on
      nokogiri (< 1.13, >= 1.12.5)

Trying to upgrade nokogiri, however, drops Ruby 2.5 support: https://rubygems.org/gems/nokogiri/versions/1.13.0

[jrgriffiniii]

ced442e starts to finally advance this towards a better resolution. I am afraid that I needed to either replace the Faraday stubs outright with RSpec or default to lamprey from rdf/ldp.

[jrgriffiniii]

https://github.com/samvera/ldp/compare/allow-ruby-3.0?expand=1 only has 5 integration tests failing for this solution.

[jrgriffiniii]

I have discovered a few new bugs with this, but this has required me to test against a containerized deployment of fcrepo4: https://github.com/samvera/ldp/compare/allow-ruby-3.0-wip?expand=1

There are some outlying WebMock stubs triggering errors, but hopefully resolving this should require minimal effort.