Fix the MySQL version in gitlab-ci.yml

What does this MR do?

Sets a fixed version of mysql to 5.7.24, to work around this a bug that our specs were depending on: From the mysql 5.7.25 changelog: https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-25.html

CREATE TABLE ... SELECT could create date columns with “zero” date default values when it should have created them without a default value. (Bug #28022129)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9323

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer