Use issuesCount field when fetching board issues count [RUN AS-IF-FOSS]

What does this MR do?

(the change affects issue boards behind the feature flag :graphql_board_lists without any visual change)

• Use issuesCount on BoardList for performance

More context: #332661 (comment 592786458)

count field serves for getting total count of records (which would be returned w/o pagination). This field is implemented by this generic type - https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/graphql/types/countable_connection_type.rb.

The above change requires the following change:

• Use issueFilters argument for specifying the filters for issues on BoardList.

Screenshots (strongly suggested)

No user facing changes.

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.
• [-] This change is backwards compatible across updates, or this does not apply.

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.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #332661 (closed)