Fix CurseForge JavaVersion for Java 1.8

[MattSturgeon]

Using JavaVersion.toString() returns "1.8" for JAVA_1_8, however modpublisher expects "Java 8".

We can instead use JavaVersion.getMajorVersion().

I've tested this using following gradle snippet:

JavaVersion.values().each {
  logger.warn "Java ${it} - ${it.getMajorVersion()}"
}

Which outputs the following logs:

Log

``` Java 1.1 - 1 Java 1.2 - 2 Java 1.3 - 3 Java 1.4 - 4 Java 1.5 - 5 Java 1.6 - 6 Java 1.7 - 7 Java 1.8 - 8 Java 9 - 9 Java 10 - 10 Java 11 - 11 Java 12 - 12 Java 13 - 13 Java 14 - 14 Java 15 - 15 Java 16 - 16 Java 17 - 17 Java 18 - 18 Java 19 - 19 Java 20 - 20 Java 21 - 21 Java 22 - 22 Java 23 - 23 Java 24 - 24 Java 25 - 25 Java 26 - 26 ```