fix: Add serverpod migrations table.

[SandPod]

Changes

• Defines a Serverpod table for the serverpod_migrations table allowing us to use our deserialization and query support.
• Removes the unsafe sql queries used to create and deserialize the serverpod_migrations table.

Pre-launch Checklist

• I read the Contribute page and followed the process outlined there for submitting PRs.
• This update contains only one single feature or bug fix and nothing else. (If you are submitting multiple fixes, please make multiple PRs.)
• I read and followed the Dart Style Guide and formatted the code with dart format.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///), and made sure that the documentation follows the same style as other Serverpod documentation. I checked spelling and grammar.
• I added new tests to check the change I am making.
• All existing and new tests are passing.
• Any breaking changes are documented below.

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Breaking changes

If you have done any breaking changes, make sure to outline them here, so that they can be included in the notes for the next release.