Allow filtering by multiple negated milestones

What does this MR do and why?

Fixes a 500 when you select multiple NOT milestone filters in the issues list

Related to #358970 (closed)

• Old query: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/10811/commands/38915
• New query (with multiple milestones): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/10811/commands/38916

Query plans are the same, only the filter condition changed.

MR acceptance checklist

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Related to #358970 (closed)