"any milestone" is inconsitent in issue boards

Summary

Selecting "any milestone" has different meaning in issue boards.

The behavior described here can only be reproduced on gitlab.com after https://gitlab.com/gitlab-org/release/tasks/issues/906 got deployed.

This issue is similar to https://gitlab.com/gitlab-org/gitlab-ee/issues/13414.

Steps to reproduce

• create issue board that contains issues with milestone and without
• use filter bar and board scope and select "any milestone"

What is the current bug behavior?

Selecting Milestone:Any in the filter bar excludes issues with no milestone:

Selecting Any Milestone in the board scope includes issues with no milestone:

Side note: The issues API also excludes issues with no milestone for milestone=any (see also https://gitlab.com/gitlab-org/gitlab-ce/issues/65676).

What is the expected correct behavior?

"any milestone" behaves the same for both cases.

Proposal

• Rename the existing filter option to 'Don't filter'
• In future iterations, add an 'Any' option that has the same functionality as Lists