meta: add legal boilerplate to PR template

[dingsdax]

Added legal boilerplate to clarify contribution rights b/c CLI is FSL

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### Fixes

- add legal boilerplate to PR template ([#3089](https://github.com/getsentry/sentry-cli/pull/3089))

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[szokeasaurusrex]

Nice! I just wonder whether we should have some CI check to ensure that this message is intact for non-employees and reject the PR otherwise. Could be helpful on other repos as well.

This would be a separate PR though

[dingsdax]

I don't think this is necessary. Upon opening a PR an external contributor reads this language, there needs to be a conscious action to remove it. I don't think we have such a check for our other FSL licensed repos either.

[szokeasaurusrex]

it is possible that they miss the template if they are opening the PR from gh CLI, which may become a more frequent use case with agentic coding workflows

[dingsdax]

still edge case territory for me. afaik GH CLI respect PR templates unless you override with "gh pr create --body", but that might be a nice addition to our Dangerfile … fine lets create a backlog issue