chore(PULL_REQUEST_TEMPLATE.md): note impact on payment handler

[Kronik-Lilus]

closes #???

The following tasks have been completed:

• [ ] Confirmed there are no ReSpec errors/warnings.
• [ ] Added Web platform tests (link)
• [ ] added MDN Docs (link)

Implementation commitment:

• [ ] Safari (link to issue)
• [ ] Chrome (link to issue)
• [ ] Firefox (link to issue)
• [ ] Edge (public signal)

Impact on Payment Handler spec?