HealthUtils - update

[ShaneBeee]

Description

• Removed some really old code that was used before MC 1.6 (prior to MC 1.6, the damageable classes methods used ints, 1.6 introduced using doubles. The reflection methods were introduced by Njol years ago to help handle the difference. Since we don't support below 1.9.4 they are most definitely not needed)
• Updated the java doc notes
• Updated to use attributes to make this class more future proof in the event the deprecated Damageable#getMaxHealth and Damageable#setMaxHealth methods are removed in future Spigot releases
• I have tested this on both 1.9.4 and 1.14.4, everything works swimmingly.

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Target Minecraft Versions: any
Requirements: none
Related Issues: none

Special Note:

This file was using CRLF, I wanted to switch it to LF but figured the PR would be hard to see what was changed.
If this is to be approved and you guys so choose, I can easily switch it to LF and push before merging.

[bensku]

Good call on not changing line endings. Now that we're changing entity damage code, testing that everything still works (a misc test) would be great.

[ShaneBeee]

Good call on not changing line endings. Now that we're changing entity damage code, testing that everything still works (a misc test) would be great.

Okay I added the health utils test for a pig, ran it using gradlew skriptTest and the test passed for all three versions