Fix incorrect invulnerability damage reduction (#11599)

Fixes incorrect spigot handling of the invulnerability damage
reduction applied when an already invulnerable entity is damaged with a
larger damage amount than the initial damage.
Vanilla still damages entities even if invulnerable if the damage to be
applied is larger than the previous damage taken. In that case, vanilla
applies the difference between the previous damage taken and the
proposed damage.

Spigot's damage modifier API takes over the computation of damage
reducing effects, however spigot invokes this handling with the initial
damage before computing the difference to the previous damage amount.
This leads to the reduction values to generally be larger than expected,
as they are computed on the not-yet-reduced value.
Spigot applies these reductions after calling the EntityDamageEvent and
*then* subtracts the previous damage point, leading to the final damage
amount being smaller than expected.

This patch cannot simply call the EntityDamageEvent with the reduced
damage, as that would lead to EntityDamageEvent#getDamage() returning
the already reduced damage, which breaks its method contract.
Instead, this patch makes use of the DamageModifier API, implementing
the last-damage-reduction as a DamageModifier.

patches/server/0202-Expand-Explosions-API.patch

@@ -6,7 +6,7 @@ Subject: [PATCH] Expand Explosions API
 Add Entity as a Source capability, and add more API choices, and on Location.
 
 Co-authored-by: Esoteric Enderman <90862990+EsotericEnderman@users.noreply.github.com>
-Co-authored-by: Bjarne Koll <lynxplay101@gmail.com>
+Co-authored-by: Bjarne Koll <git@lynxplay.dev>
 
 diff --git a/src/main/java/net/minecraft/server/level/ServerLevel.java b/src/main/java/net/minecraft/server/level/ServerLevel.java
 index 03161ef12960cab3b9c81c190dd808a9c5c476e1..f0fc08bcc2988277b6a5e3107e6fc5c89bb67b93 100644