Add privacy notice to pr and issue templates

[RaphaelVogel]

This PR adds a privacy notice to pr and issue template files to remind users not to include company internal information.

Files modified:

• .github/ISSUE_TEMPLATE/bug.md
• .github/ISSUE_TEMPLATE/feature.md
• .github/ISSUE_TEMPLATE/flaking-test.md
• .github/pull_request_template.md

[gardener-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[gardener-prow]

@RaphaelVogel: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-extension-networking-cilium	fe380b7	link	true	/test pull-extension-networking-cilium

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see our testing guideline for how to avoid and hunt flakes.

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