Bump minimum SQLite version to 3.8 for Rails 6

Hi,

I'd like to propose that Rails 6 requires SQLite 3.8 or higher.

These OS versions have SQLite 3.8 or higher by default.

- macOS 10.10 (Yosemite) or higher - Ubuntu 14.04 LTS or higher

There was a similar discussion for Rails 5 two years ago, yes I requested that one. https://groups.google.com/forum/#!topic/rubyonrails-core/Mm4ZN_g1RXk

Since two years passed since my previous request, Ubuntu 12.04 LTS support has been dropped and newer macOS versions have been released. I think it is good time to bump the minimum version of SQLite for Rails 6.

Also, bumping SQLite version 3.8 will allow supporting `alter_table` method for the table with foreign keys by this pull request:

"Disable foreign keys during `alter_table` for sqlite3 adapter" https://github.com/rails/rails/pull/32865 requires SQLite 3.8 to support `defer_foreign_keys` pragma.

Please review my proposal. I am ready to open a pull request.

Thanks,

This proposal has been implemented for Rails 6 by this pull request Bump minimum SQLite version to 3.8 by yahonda · Pull Request #32923 · rails/rails · GitHub

Thanks,