Bump com.github.johnrengelman.shadow from 7.1.2 to 8.1.1

Bumps com.github.johnrengelman.shadow from 7.1.2 to 8.1.1.

---
updated-dependencies:
- dependency-name: com.github.johnrengelman.shadow
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

build.gradle

@@ -1,6 +1,6 @@
 plugins {
     id 'java'
-    id 'com.github.johnrengelman.shadow' version '7.1.2'
+    id 'com.github.johnrengelman.shadow' version '8.1.1'
 }
 
 group 'it.osm.gtfs'