docs(redis): document Valkey compatibility

[atao2004]

Pull Request type

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• WHOSUSING.md
• Other (please describe): Documentation

NOTE: Please remember to run ./gradlew spotlessApply to fix any format violations.

Changes in this PR

Documents Valkey as a compatible persistence backend for Conductor. Conductor's Redis
backend (Jedis 6.0.0, Redisson 3.22.0) communicates using standard RESP commands only —
no Redis modules (RedisSearch, RedisJSON, etc.) are used — so Valkey works as a drop-in
replacement, including managed offerings like AWS ElastiCache for Valkey and GCP
Memorystore for Valkey. Today this compatibility is undocumented, so users have to
discover and verify it themselves.

This PR is docs-only, scoped down from the original proposal in #1403 per maintainer
feedback ("Readme updates and documentation can be added for Valkey... we need not go
in detail in adding Docker Compose or integration instructions with specific cloud
providers"). No Docker Compose samples, config templates, or CI changes are included.

• Adds a "Valkey compatibility" section to docs/documentation/advanced/redis.md,
  stating the RESP-only compatibility, that no new configuration is required beyond
  pointing conductor.redis.hosts at the Valkey endpoint, and that Redisson officially
  supports Valkey 7.2.5 and above.
• Adds a one-line pointer to this section in README.md's Backend Configuration table.

No code, configuration, or test changes. No existing Redis documentation, samples, or
behavior is modified.

Alternatives considered

The original issue proposed a Docker Compose sample, standalone/cluster/sentinel config
templates, an AWS ElastiCache walkthrough, a Redis→Valkey migration guide, and a CI job
validating the existing test suite against Valkey. The maintainer asked to keep this to
README/documentation only, so those pieces are deliberately left out of this PR rather
than deferred to a follow-up — see #1403 for that scoping discussion.

Testing

This is a documentation-only change; no code or runtime tests are required. The fork PR
was reviewed and approved, including verification of the Redisson compatibility link and
version claim.

Reviewer comments addressed

• Replaced the unrelated Redisson Pro data-partitioning link with the Redisson repository README.
• Updated the compatibility wording to match Redisson's documented "from 7.2.5" support range.

Fixes #1403