Switch to Spigot API and bump version to 1.17.

Spigot has decided to discontinue distribution of the Bukkit artifacts,
which means Bukkit "doesn't exist" after Minecraft 1.15. This commit
takes the plunge on that change and moves to the Spigot API rather than
the Bukkit API. Goodbye, Bukkit. Hello, Spigot.

The version is also bumped to 1.17 to give way to new features and bug
fixes otherwise blocked by staying on 1.13. Unfortunately, this brings
with it a swath of potential issues with version compatibility going
forward, because backwards compatibility in the API sometimes takes a
back seat, such as with the Material and EntityType enums. This commit
specifically changes some references that break compatibility with older
server versions, but compatibility with those is reinstated later.

Finally, the Spigot repository is changed to match the consensus about
which path on the Spigot Nexus instance to use. This seems to be what
everyone else is doing, so `bandwagon.jumpOn()`.

pom.xml

@@ -74,7 +74,7 @@
     <!-- Spigot repo -->
     <repository>
       <id>spigot-repo</id>
-      <url>https://hub.spigotmc.org/nexus/content/groups/public/</url>
+      <url>https://hub.spigotmc.org/nexus/content/repositories/snapshots/</url>
     </repository>
 
     <!-- JitPack for Vault -->
@@ -85,11 +85,11 @@
   </repositories>
 
   <dependencies>
-    <!-- Spigot-Bukkit API -->
+    <!-- Spigot API -->
     <dependency>
-      <groupId>org.bukkit</groupId>
-      <artifactId>bukkit</artifactId>
-      <version>1.13-R0.1-SNAPSHOT</version>
+      <groupId>org.spigotmc</groupId>
+      <artifactId>spigot-api</artifactId>
+      <version>1.17-R0.1-SNAPSHOT</version>
       <scope>provided</scope>
     </dependency>