Fix system name for MySQL

[bgrainger]

Changes

MySQL should be identified as "mysql", not "microsoft.sql_server".

This fixes a regression introduced in https://github.com/open-telemetry/semantic-conventions/commit/a86a51c9325a2ed43b4dc363b008b4bc485b7158#diff-8b7054e134a55e0a1ec2dbd60371b69ee98213984c9973587f428e8f3695ec7a.

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [ ] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [ ] schema-next.yaml updated with changes to existing conventions.