Fix single quote escapes on default generated MySQL columns

[yash]

Closes #50732

Motivation / Background

MySQL 5.7.5+ supports generated columns, which can be used to create a column that is computed from an expression. If single quotes are used in the definition of a default generated column, the schema dump will escape the quotes twice.

In a scenario such as the following:

create_table :people, force: true do |t|
  t.string :first_name
  t.string :last_name
  t.string :full_name, default: -> { "(concat(`first_name`,_utf8mb4'-',`last_name`))" }
end

We want the schema dump to look like this:

t.string "full_name", default: -> { "(concat(`first_name`,_utf8mb4'-',`last_name`))" }

And not this, which is how it now it is now:

t.string "full_name", default: -> { "(concat(`first_name`,_utf8mb4\\'-\\',`last_name`))" }

See the related issue for more.

Detail

This pull request changes the schema statement generator under the elsif block for a column with DEFAULT_GENERATED in the metadata. In the changes, we simply gsub the escapes as we do in other parts of the class.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message.
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[byroot]

Backported to 7.1 and 7.0