Align font-weight of heading-classes with their related elements

What does this MR do?

Currently the heading-level css classes (.h1 - .h6) have a different font-weight (500) than their related heading elements (600).

This MR aligns the the font-weight and uses $gl-font-weight-bold (600) to set it.

The change will affect the following components:

ee/app/assets/javascripts/status_page_settings/components/settings_form.vue
65:      <h3 ref="sectionHeader" class="h4">

ee/app/assets/javascripts/dependencies/components/app.vue
163:      <h2 class="h4 mb-1">

ee/app/assets/javascripts/license_compliance/components/app.vue
87:    <h2 class="h4">

ee/app/assets/javascripts/security_configuration/components/app.vue
75:      <h2 class="h4 my-3">

ee/app/assets/javascripts/security_dashboard/components/project_list.vue
38:      <h4 class="h5 font-weight-bold text-secondary border-bottom mb-3 pb-2">

ee/app/assets/javascripts/threat_monitoring/components/threat_monitoring_section.vue
104:    <h4 class="h4">{{ title }}</h4>
105:    <h5 class="h5">{{ subtitle }}</h5>

ee/app/assets/javascripts/threat_monitoring/components/app.vue
141:    <h2 class="h3 mb-1">

ee/app/views/projects/issues/_related_issues.html.haml
8:          %h3.card-title.mt-0.mb-0.h5 

app/views/profiles/chat_names/new.html.haml
3:  %p.h4

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #12530 (closed)