avoid linking keywords in PR template

Certain phrasing in a PR can "link" it to a particular issue. Merging a PR that is "linked" to an issue will automatically close that issue. This can be handy, but it can also be problematic if the issue is closed unintentionally, like when a PR does not completely cover all of the points made in the issue. For examples of this, see: https://github.com/cmu-delphi/covidcast-indicators/issues/1676#issuecomment-2118194957 and https://github.com/cmu-delphi/covidcast-indicators/issues/1973#issuecomment-2249126852 . The automatic closing behavior can not be disabled, though it is a commonly requested feature. This PR removes those linking keywords from the template to reduce the possibility of an undesired linking.

cmu-delphi / delphi-epidata