Add jira base URL to PR template for ease-of-use

[internetisaiah]

Why are you making this change? (required)

Figured it would be helpful to have the base link in the following part of the PR template so its easy to add the link without needing to copy+paste:

<!--
For related GitHub issues and features, add "Fixes #ISSUE_NUMBER".
For Jira tickets, add the ticket number and a link to the ticket, like: [BD-NUMBER](https://jira.braze.com/browse/BD-NUMBER).
-->

Note: this does not expose any sensitive information.

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