[Improvement] Use SQL prepared statements in JdbcDatabaseOperations.java and PostgreSqlSchemaOperations.java

### What would you like to be improved?

In the exist method the SQL string can be changed to use safer prepared statements.

### How should we improve?

generateDatabaseExistSql woudl need to change as well.

Start in JdbcDatabaseOperations.java and PostgreSqlSchemaOperations.java by tracing the exist method and its SQL generation. Check how generateDatabaseExistSql is used, then update the SQL handling to use prepared statements and run the existing tests covering these database operations.

Written by the indexing model from the issue text.