MINOR: Cleanup Connect Module (3/n)

[sjhajharia]

Now that Kafka support Java 17, this PR makes some changes in connect
module. The changes in this PR are limited to only some files. A future
PR(s) shall follow.

The changes mostly include:

• Collections.emptyList(), Collections.singletonList() and Arrays.asList() are replaced with List.of()
• Collections.emptyMap() and Collections.singletonMap() are replaced with Map.of()
• Collections.singleton() is replaced with Set.of()

Modules target: runtime/src/test