GitLab for Jira - Update copy around linking of groups

What does this MR do and why?

Update copy around linking of groups:

• Update references of "namespace" to "group" to avoid confusion for users.
• Update "add" to "link".
• Use a consistent translation namespace "JiraConnect" throughout the app.

Changelog: changed

Screenshots or screen recordings

Description	Before	After
Button
Modal
Modal empty

How to set up and validate locally

https://docs.gitlab.com/ee/development/integrations/jira_connect.html#simple-setup

To simulate the empty state, apply this diff:

diff --git a/app/assets/javascripts/jira_connect/subscriptions/components/add_namespace_modal/groups_list.vue b/app/assets/javascripts/jira_connect/subscriptions/components/add_namespace_modal/groups_list.vue
index a4b728335c59..fc9c918178d4 100644
--- a/app/assets/javascripts/jira_connect/subscriptions/components/add_namespace_modal/groups_list.vue
+++ b/app/assets/javascripts/jira_connect/subscriptions/components/add_namespace_modal/groups_list.vue
@@ -62,7 +62,7 @@ export default {
           const { page, total } = parseIntPagination(normalizeHeaders(response.headers));
           this.page = page;
           this.totalItems = total;
-          this.groups = response.data;
+          // this.groups = response.data;
         })
         .catch(() => {
           this.errorMessage = s__('Integrations|Failed to load namespaces. Please try again.');

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 #331432 (closed)