Update the jira_issue_key_regex to conform to the Jira issue key default format

See #352411 (comment 1271002431).

Context

The project key is a unique identifier, automatically created with each project, that makes up the first part of each item's Unique ID, for example Jira issue key.

The default format for Jira issue key is therefore two or more uppercase letters ( [A-Z][A-Z]+ ), followed by a hyphen and the issue number (e.g. "TEST-123").

Current behaviour

Currently GitLab's code tries to pick up Jira issue keys from strings like "thisNOT-123", in this case assuming that "NOT-123" would be Jira issue key. That is not correct, as Jira issue key starts with uppercase letter and contains only upper case letters, hypen and numbers.

The current incorrect regex: /(?!CVE-\d+-\d+)[A-Z][A-Z_0-9]+-\d+/

Proposed solution

We should pick up Jira issue keys from String only if they conform to the Jira issue key default format of two or more uppercase letters ( [A-Z][A-Z]+ ), followed by a hyphen and the issue number (e.g. "TEST-123").