Simplify code: remove Java 8 workarounds, reflection and compatibility paths

With a single compilation target, the compatibility layers can go.

• Replace Class.forName presence checks for optional dependencies (slf4j in Loggers, AWS SDK in AwsIamCredentialPlugin) with ServiceLoader or requires static module resolution where possible.
• Use switch expressions, records for immutable value holders (host addresses, prepare results, column definitions), text blocks in tests, String.repeat, List.of / Map.of, Stream.toList(), and InputStream.readNBytes / transferTo in the stream classes.
• Replace hand-rolled utilities that now exist in the JDK.
• Review all @SuppressWarnings and -Xlint exclusions kept for Java 8.