Revert: Grant migrator privileges even when the user already exists

[CarolineDenis]

Fixes #8305

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list
• Add automated tests
• Add a reverse migration if a migration is present in the PR
• Add migration function to

  specify7/specifyweb/specify/management/commands/run_key_migration_functions.py

  Line 50 in ea04665

  def fix_schema_config(stdout: WriteToStdOut | None = None):

Testing instructions

1. Check that you have a specify_migrator* and specify_user* already created in MySQL.

• *The users may not be of those names. They need to match the MIGRATOR_NAME and APP_USER_NAME in the .env file.

2. Edit the .env file so that the DATABASE_NAME is one that does not exist yet.
3. Build/rebuild your Docker containers.

• Check the specify7 container logs and make sure there are no migration errors.

4. Open the application in your browser.

• Make sure that it's opening the Specify 7 initial database setup screen.

5. Stop the containers.
6. Edit the .env file so that the DATABASE_NAME is one that does exist and was not previously opened in v7.12.

• You may run into migration errors while testing an old database.

7. Build/rebuild your Docker containers.

• Check the specify7 container logs and make sure there are no migration errors.

8. Open the application in your browser.

• Make sure that it's opening the Specify 7 login page.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-8305

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[combs-a]

I wrote these testing instructions, but they may need to be added onto if anyone spots any additional coverage needed!

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New database:

• Check the specify7 container logs and make sure there are no migration errors.
• Make sure that it's opening the Specify 7 initial database setup screen.

Previously existing database (used pre v7.12):

• Check the specify7 container logs and make sure there are no migration errors.
• Make sure that it's opening the Specify 7 login page.

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New database worked as expected.

Was able to create a new database with no issues, but ran into an issue with the Specify app user when attempting to open an old database. Used the same database as I did in the originally reported issue.