Stable 25.8 - Regression tests: use release branch instead of commit hash

[CarlosFelipeOR]

Changelog category (leave one):

• CI Fix or Improvement (changelog entry is not required)

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• Aarch64 tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• OAuth (5m)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• S3 Export (2h)
• Swarms (30m)
• Tiered Storage (2h)

[github-actions]

Workflow [PR], commit [6e0a359]

[mkmkme]

No need to backport to Antalya 25.8 -- yaml_additional_templates.py is handled by #2121, and pull_request.yml and master.yml are handled by #1715