Add simple dropdown component and change severity filter to use it

What does this MR do and why?

This MR adds simple_dropdown.vue and updates severity_filter.vue to use it. There are a bunch of vulnerability report filters that repeat the same boilerplate code, and simple_dropdown.vue refactors them into a reusable component.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383346 (closed)

.rubocop_todo/layout/first_array_element_indentation.yml

----
-# Cop supports --autocorrect.
-Layout/FirstArrayElementIndentation:
-  Exclude:
-    - 'spec/lib/gitlab/github_import/importer/issues_importer_spec.rb'
-    - 'spec/lib/gitlab/search/found_blob_spec.rb'
-    - 'spec/models/ci/runner_version_spec.rb'
-    - 'spec/models/repository_spec.rb'
-    - 'spec/requests/api/task_completion_status_spec.rb'
-    - 'spec/services/security/merge_reports_service_spec.rb'
-    - 'spec/simplecov_env.rb'
-    - 'spec/support/atlassian/jira_connect/schemata.rb'
-    - 'spec/support/capybara.rb'
-    - 'spec/support/helpers/project_template_test_helper.rb'
-    - 'spec/support/helpers/test_env.rb'
-    - 'spec/support/helpers/usage_data_helpers.rb'
-    - 'spec/support/matchers/exceed_query_limit.rb'
-    - 'spec/support/migrations_helpers/vulnerabilities_findings_helper.rb'
-    - 'spec/support/prometheus/additional_metrics_shared_examples.rb'
-    - 'spec/support/shared_contexts/policies/group_policy_shared_context.rb'
-    - 'spec/support/shared_examples/graphql/label_fields.rb'
-    - 'spec/support/shared_examples/lib/gitlab/middleware/multipart_shared_examples.rb'
-    - 'spec/support/shared_examples/requests/api/repository_storage_moves_shared_examples.rb'
-    - 'spec/views/projects/issues/_related_branches.html.haml_spec.rb'
-    - 'tooling/lib/tooling/helm3_client.rb'