Update dependency rails to v6

[renovate]

This PR contains the following updates:

Package	Update	Change
rails	major	~> 5.2.2 -> ~> 6.0.0

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[renovate]

⚠️ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻️ Renovate will retry this branch, including artifacts, only when one of the following happens:

• any of the package files in this branch needs updating, or
• the branch becomes conflicted, or
• you check the rebase/retry checkbox if found above, or
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The artifact failure details are included below:

File name: Gemfile.lock

ruby 2.3.7p456 (2018-03-28 revision 63024) [x86_64-linux]
Successfully installed bundler-1.17.2
1 gem installed
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
Fetching gem metadata from https://rubygems.org/............
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "ruby�":
  In Gemfile:
    ruby� (~> 2.3.7.0)

    rails (~> 6.0.0) was resolved to 6.0.2.1, which depends on
      ruby� (>= 2.5.0)