[chore] Fix sourcecode-release workflow permissions (#13301)

#### Description

During the v0.129.0 release process, the sourcecode-release.yaml
workflow
[failed](https://github.com/open-telemetry/opentelemetry-collector/actions/runs/15973270037/job/45049168039),
during the step that [updates the "next release"
milestone](https://github.com/open-telemetry/opentelemetry-collector/blob/8c7e0b7708eeb02f012282f74c6723a58f9cbf2a/.github/workflows/sourcecode-release.yaml#L56).
I believe this was because of [this
change](https://github.com/open-telemetry/opentelemetry-collector/pull/13240/files#diff-ac51d4c0a4ced767627f2ed87af1fe93f5eba40f8de1d0a4dd5b1b99914e1d88),
which accidentally removed the workflow's permission to modify issue
milestones.

This PR adds back the appropriate permission explicitly.

([→ Doc page showing that "issues" is the appropriate scope for
modifying
milestones](https://docs.github.com/en/rest/issues/milestones?apiVersion=2022-11-28#update-a-milestone))

Author: jade-guiton-dd

.github/workflows/sourcecode-release.yaml

@@ -10,6 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: write # Grant write permissions to repository contents
+      issues: write # Grant write permissions to PR milestones
     steps:
       - name: Checkout Repo
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2