chore: migrate gem to tag-based GLQL dependency

What does this MR do and why?

This change improves version management between the gem and GLQL core by using explicit tag-based dependencies instead of tracking the main branch.

Changes:

• Update gem's Cargo.toml to use tag-based dependency (tag = "v0.20.9") instead of branch-based (branch = "main")
• Modify release script to automatically update the tag reference when creating a new release
• Update config.toml comment to reflect tag-based approach

Benefits:

• Explicit version coupling: gem v0.21.0 clearly references GLQL v0.21.0
• Version stability: gem won't pick up unrelated changes from main branch
• Independent release timing: gem can stay on older GLQL versions if needed
• Better reproducibility: builds always fetch the exact same GLQL version
• Local development unchanged: still uses patch override for testing

Note: GLQL must still be tagged before the gem can reference it, but this approach provides clearer version relationships and more control over which GLQL version the gem depends on.