BoostSync-1.0.0.jar

Target Paper API for 1.20 through 26.2 support

Switch the build from Spigot API (1.19.4) to Paper API, compiling against
the 1.20.1 baseline so a single jar runs on every version from 1.20 up
through the latest 26.2 release.

- pom.xml: replace org.spigotmc:spigot-api with io.papermc.paper:paper-api
  1.20.1-R0.1-SNAPSHOT; swap the Spigot/BungeeCord repos for the PaperMC repo.
- plugin.yml: raise api-version from 1.13 to 1.20.

Only public Bukkit/Paper API is used (no NMS/internal names), so the plugin
is unaffected by Mojang dropping obfuscated jars in 26.1+. Java 17 bytecode
keeps compatibility with both Java 17 (1.20.x) and Java 21+ (1.20.5 onward)
runtimes.