Support Redmine v5.1

[otegami]

What I did

• Adding needed changes for Redmine 5.1
  • Prepare sample database.ymls for MySQL and PostgreSQL
  • Use db:schema:dump from Rails v6 instead of db:structure:dump
    • ref: Combine and deprecate rails db:structure:{dump,load} tasks into rails db:schema:{dump,load} rails/rails#39470
• Add CI for Redmine v5.1

Note

• There are no breaking changes from 5.0
  • https://www.redmine.org/news/142

What I didn't

• Interact with all the provided features
  • I determined there were no errors since the CI passed
• Update README about Redmine version from 5.0 to 5.1
• Update News.md

[otegami]

I will check whether I have forgotten anything or not. And then I will ask for reviews.

[kou]

Thanks!

(We should have referred #116 in the PR description...)