Add minimum versions of databases to the docs (#17080)

We do not currently state the minimum versions of databases we support.

This PR sets them to:
* MySQL >=5.7
* Postgres >=10
* MSSQL >=2008R2 SP3

Signed-off-by: Andrew Thornton <art27@cantab.net>

docs/content/page/index.en-us.md

@@ -69,10 +69,10 @@ Windows, on architectures like amd64, i386, ARM, PowerPC, and others.
 - Logging
 - Configuration
     - Databases
-        - MySQL
-        - PostgreSQL
+        - MySQL (>=5.7)
+        - PostgreSQL (>=10)
         - SQLite3
-        - MSSQL
+        - MSSQL (>=2008R2 SP3)
         - TiDB (experimental, not recommended)
     - Configuration file
         - [app.ini](https://github.com/go-gitea/gitea/blob/master/custom/conf/app.example.ini)