CI: Tell dependabot to update GH Actions versions

[olleolleolle]

This PR adds a configuration to tell Dependabot to update the versions used for some well-known GitHub Actions, such as checkout, in order to avoid deprecation warnings when those get too old. Documentation

In addition,

• fix the permissions-setting step to point to the correct file
• avoid Gemfile-related warnings about gemified standard gems that

Not done here:

• make linting pass

Current test failing with

  1) Ferrum::Browser::Binary#find finds one binary
     Failure/Error: raise ProcessTimeoutError.new(timeout, output)
     Ferrum::ProcessTimeoutError:
       Browser did not produce websocket url within 25 seconds, try to increase `:process_timeout`. See https://github.com/rubycdp/ferrum#customization
     # ./lib/ferrum/browser/process.rb:170:in 'Ferrum::Browser::Process#parse_ws_url'
     # ./lib/ferrum/browser/process.rb:104:in 'Ferrum::Browser::Process#start'
     # ./lib/ferrum/browser.rb:277:in 'Ferrum::Browser#start'
     # ./lib/ferrum/browser.rb:136:in 'Ferrum::Browser#initialize'
     # ./spec/spec_helper.rb:48:in 'Class#new'
     # ./spec/spec_helper.rb:48:in 'block (2 levels) in <top (required)>'

Perhaps this is an issue with "the browser is now called something else". I'll see if I can get a hold of this.