API: SQL class definitions renaming

- renaming of PandasSQLAlchemy to SQLDatabase and PandasSQLTable to SQLTable
- renaming of SQLDatabase.read_sql to read_query (to be consistent with read_query/read_table)
- legacy -> fallback
Merge pull request #8440 from jorisvandenbossche/sql-ooapi-naming

API: SQL class definitions renaming