Port to Paper 26.3 (2.1.0)

Built against paper-api 26.3.build.140-beta on Java 25, api-version 26.3,
version 2.1.0 so the jar reports the number Modrinth shows. Release notes
are in the 2.1.0 section of the Modrinth changelog.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/main/resources/plugin.yml

@@ -1,7 +1,7 @@
 name: CommunityMarket
 version: '${project.version}'
 main: pt.henrique.communityMarket.CommunityMarket
-api-version: "26.2"
+api-version: "26.3"
 description: A GUI-only marketplace plugin for fixed-price listings and auctions
 author: Henrique
 website: https://github.com/henriquescrrrr/carrageis-communitymarket