Stable 26.8 - Version logic rework

[strtgbb]

Upstream version logic has changed in 26.x, rework our customizations to minimize conflicts and improve functionality.

• VERSION_STRING and VERSION_DESCRIBE are common merge conflicts, they are now automated.
• Fix and improve tag validity checks. Get the flavour from tag so the altinitytest flavour remains supported.
• • Recognize new prelrease labels antalyatest and altinityfipstest alongside the existing altinitytest, which remains the default for local builds.
• Recognize new prerelase labels antalyatest and altinityfipstest alongside the existing altinitytest, which remains the default for local builds.
• Derive VERSION_FLAVOUR and first digit of VERSION_TWEAK from branch name so that it no longer gets forgotten when CICD gets rebased from stable to antalya and vice versa.

Next steps:

• Derive VERSION_TWEAK by checking release history. This would remove the need for manual bumps.

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
• Unit 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)
• CAS (content-addressed storage; Antalya only)
• 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 [68f16d5]