Build.yaml is currently
https://raw.githubusercontent.com/malte0811/FerriteCore/main/.github/workflows/build.yaml
Build.yaml should be
name: Build and publish as artifact
on:
push:

jobs:
run:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v2
- name: Set up JDK 16
uses: actions/setup-java@v1
with:
distribution: 'adopt'
java-version: 16
- uses: actions/cache@v2
with:
path: |
~/.gradle/caches
~/.gradle/wrapper
key: ${{ runner.os }}-gradle-${{ hashFiles('/.gradle', '/gradle-wrapper.properties') }}
restore-keys: |
${{ runner.os }}-gradle-
- name: Build
run: ./gradlew build
- name: Upload artifacts
uses: actions/upload-artifact@v2
with:
name: build-artifacts
path: |
forge/build/libs/ferritecore--forge.jar
fabric/build/libs/ferritecore--fabric.jar

That's for the main branch, which is on 1.16.5 (until Forge gets Mixins on 1.17 at least). The 1.17 build.yaml already uses Java 16.