Switch to OIDC Federation Service instead of GitHub App (#222)

Currently, the [Gardener GitHub-Actions App](https://github.com/apps/gardener-github-actions)
is used to provide more privileged access than available via the default `GITHUB_TOKEN`, for
example to circumvent branch protection rules (GitHub Apps can be configured as bypassers) or
cross repository privileges. To prevent sharing the GitHub App secret with each and every
repository/workflow which requires usage of it, the
[GitHub OIDC Federation Service](https://github.com/gardener/github-oidc-federation) has been
developed. In essence, it holds the credentials for a central GitHub App and creates short-lived
access tokens with a configured scope based on a centrally configured OIDC configuration. See
related changes which have been necessary for this repository:

- gardener/.github-oidc@03bcd3a

Signed-off-by: Jonas Brand (i538859) <j.brand@sap.com>

Author: 8R0WNI3

.github/workflows/build.yaml

@@ -17,7 +17,7 @@ jobs:
     with:
       mode: ${{ inputs.mode }}
     permissions:
-      contents: read
+      id-token: write
 
   oci-images:
     name: Build OCI-Images

.github/workflows/non-release.yaml

@@ -13,15 +13,14 @@ jobs:
       mode: snapshot
     secrets: inherit
     permissions:
-      contents: write
+      contents: read
       packages: write
       id-token: write
 
   component-descriptor:
     uses: gardener/cc-utils/.github/workflows/post-build.yaml@master
     needs:
       - build
-    secrets: inherit
     permissions:
       id-token: write
       contents: write

.github/workflows/release.yaml

@@ -12,8 +12,9 @@ on:
 jobs:
   build:
     uses: ./.github/workflows/build.yaml
+    secrets: inherit
     permissions:
-      contents: write
+      contents: read
       id-token: write
       packages: write
     with: