Drop support for 1.14, 1.15

[ybw0014]

Description

Dropped support for Minecraft 1.14 and 1.15, since spigot caps Java version. See https://github.com/Slimefun/Slimefun4/issues/3627.

Proposed changes

• Bump api-version to 1.16 in plugin.yml.
• Remove 1.14 & 1.15 in MinecraftVersion.
• Remove version check below 1.16 (inclusive).

For tests:

• increase the major version in MinecraftVersion by two (e.g. 1.14 -> 1.16, 1.15 -> 1.17).

Related Issues (if applicable)

Resolves #3627

Checklist

• [x] I have fully tested the proposed changes and promise that they will not break everything into chaos.
• [ ] I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• [ ] I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.14.* - 1.19.*).
• [x] I followed the existing code standards and didn't mess up the formatting.
• [ ] I did my best to add documentation to any public classes or methods I added.
• [ ] I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• [ ] I added sufficient Unit Tests to cover my code.

[ybw0014]

Tested only in 1.19 and works fine.

[ybw0014]

I have no idea why CI failed

[ybw0014]

Ok, problem solved.