Bump GLQL to 0.20.12 for epic work items fix

What does this MR do and why?

Bump GLQL to 0.20.12 for epic work items fix!

See:

• What this fixes: GLQL: how to find issues with missing epics (#583341 - closed)
• How it does it: Simplify FeatureFlags implementation, and fix e... (glql!316 - merged)

Supercedes Update dependency @gitlab/query-language-rust t... (!219477 - closed).

How to set up and validate locally

1. Check out, yarn, bundle and gdk restart rails-web to run the new dependencies.
2. Try pasting the below queries into an issue comment on your GDK. They should work:

   ```glql
   query: epic = none
   ```

   ```glql
   query: epic = any
   ```

Note that epic = () is not supported — the = operator with a list on the right-hand side means "includes all in list", and only makes sense if the left-hand side can have multiple values, but there can only be one (or no) parent epic.

On the other hand, != () does work, as it means "isn't contained in list". (This might be better as not in, or a generic not operator, as it might be seen as a little confusing right now.)

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