Drop Java 8 support

[nehaev]

Loki4j is a Java 11 project from the day one. Java 8 support was added later via tooling, see #9. Now, 4 years later, I think it's time to move on and drop Java 8 support. There are several reasons for this:

1. Logback has dropped Java 8 support in its most recent major versions (1.4 and 1.5);
2. According to Maven Central stats, 5-7% of all unique downloads of Loki4j during last 12 months were Java 8 artifacts. However, only about 10% of them were the latest Loki4j version.