Remove MySQL and Sqlite deps

[NicolasDorier]

This completely removes support for MySQL and SQLite, as well as our migration code.

• Starting with 1.7.1 (Nov 2022), we started issuing deprecation warnings for MySQL and SQLite deployments.
• As of 1.7.9 (Feb 2023), we began supporting migrations from MySQL and SQLite to PostgreSQL.
• Starting with 1.9.0 (Apr 2023), using BTCPay Server with MySQL and SQLite required a special -deprecated flag.

The user base was already small, and various stacks such as Raspiblitz, Umbrel, and Embassy OS, which used MySQL, had already migrated most of their users.

By the time we release 2.0, I believe nearly two years will have elapsed since we indicated our intention to stop supporting MySQL and SQLite.

Previously, the command dotnet ef migrations add xxx used the SQLite provider to generate migration scripts, which incidentally worked for PostgreSQL as well.

This change triggered modifications to ApplicationDbContextModelSnapshot.cs, but the generated schema is nevertheless completely equivalent (this merely converted SQL standard type names to PostgreSQL-specific type names).