ci: Replace ubuntu-latest with ubuntu-slim in lightweight workflow runners

[Copilot]

Replaces ubuntu-latest with ubuntu-slim in 9 lightweight automation workflows to reduce runner startup time and resource consumption.

Changes

Updated workflows (10 runner instances):

• auto-assign-pr.yml - PR author assignment
• crowdin.yml - Translation sync
• dartdoc.yml - Documentation generation
• github-projects-ventilation.yml - Project board automation
• label.yml - PR labeling
• merge-conflict-autolabel.yml - Merge conflict detection
• release-please.yml - Release automation (2 jobs)
• semantic-pr.yml - PR title validation
• top-issues.yml - Issue tracking

Unchanged workflows:
Complex workflows requiring substantial resources (builds, tests, releases, asset generation) remain on ubuntu-latest.

jobs:
  assign-author:
-   runs-on: ubuntu-latest
+   runs-on: ubuntu-slim

Original prompt

Replace ubuntu-latest with ubuntu-slim across all listed GitHub Actions workflows to reduce runner startup time and resource consumption. Do not replace for complex workflows that may require a lot of memory. Below a list of workflows that are definitely safe (from the openfoodfacts-server, names and workflows may vary based on the repo where we're running this refactor task)

auto-assign-pr.yml
codeql-analysis.yml
crowdin-per-language.yml
crowdin.yml
generate-doc.yml
github-projects-ventilation.yml
label.yml
merge-conflict-autolabel.yml
pr_actions.yml
release-please.yml
semantic-pr.yml
top-issues.yml
translation-check.yml

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