Resolve "Remove `subscribed` from GraphQL boards list query"

What does this MR do?

Currently, GraphQL boards have a large loading time (~8-10s for 15 issues on the list). In order to improve loading times, we should remove 'unused' fields from query (we can load them later after clicking on the issue card and opening a sidebar). We start with removing subscribed field as it's already handled with Notifications widget.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #331092 (closed)