docs: drop MariaDB, pin MySQL minimum to 8.0.13, frame PostgreSQL as peer

DataJoint 2.x is officially MySQL/PostgreSQL only; MariaDB is no
longer listed as a supported backend (it was never tested in CI and
recent feature work has begun to require MySQL 8.0.13-only columns).
PostgreSQL is reframed as a peer production backend rather than "an
alternative."

- installation.md: requirements line lists MySQL 8.0.13+ and PostgreSQL 15+.
- database-backends.md: MySQL row pinned to 8.0.13.
- define-tables.md: "native MySQL/MariaDB types" → "native database types".

Author: dimitri-yatsenko

src/how-to/define-tables.md

@@ -176,7 +176,7 @@ parameters : json             # JSON data (core type, no brackets)
 
 ## Native Database Types
 
-You can also use native MySQL/MariaDB types directly when needed:
+You can also use native database types directly when needed:
 
 ```python
 definition = """

src/how-to/installation.md

@@ -78,7 +78,7 @@ See [Configure Database Connection](configure-database.md) for connection setup.
 ## Requirements
 
 - Python 3.10+
-- MySQL 8.0+ or MariaDB 10.6+
+- MySQL 8.0.13+ or PostgreSQL 15+
 - Network access to database server
 
 ## Troubleshooting

src/reference/specs/database-backends.md

@@ -9,7 +9,7 @@ DataJoint supports multiple database backends through a unified adapter architec
 
 | Backend | Minimum Version | Default Port | Status |
 |---------|-----------------|--------------|--------|
-| MySQL | 8.0 | 3306 | Production |
+| MySQL | 8.0.13 | 3306 | Production |
 | PostgreSQL | 15 | 5432 | Production |
 
 ## Configuration