Add Redis 8 to CI test matrix

[abrookins]

Problem

CI currently only tests against redis/redis-stack:latest. With Redis 8 released (May 2025) and known compatibility issues, we should explicitly test multiple Redis versions.

Current CI Config

redisstack: [ "latest" ]

Proposed Change

redisstack: [ "7.4.0-v2", "latest" ]

Or use explicit version tags:

redisstack: [ "7.4.0-v2", "8.0.0" ]

Benefits

• Catch Redis version-specific issues before release
• Ensure backwards compatibility with Redis 7.x
• Validate fixes for Redis 8 issues (like Querying over indexed bool fields fails on Redis 8, results ignore boolean expression (but it works well/as expected with Redis 7.4.x) #712)