Add 2024 security audit report and accompanying documentation

[faern]

The 2024 app audit has been completed, we have fixed the found issues, I have written responses to all the findings. This PR adds the audit report and our summary of the audit to the audit/ directory as usual.

This PR also updates changelog and known issues documents to link into the audit document where relevant. Please notify me if there are more places where it makes sense to provide cross-links.

We will blog about this audit after this PR has been merged, so the blog can link back to these documents. The plan is that this should happen today.

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This change is 

[pinkisemils]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

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audits/2024-12-10-X41-D-Sec.md line 208 at r1 (raw file):

Attacks like these are not specific to Mullvad VPN. The attack simply relies on core
internet functionality and pattern matching. The threat model defined in the report makes
it clear that a too powerful attacker who can observe and manipulate internet traffic

Nit: I'd use powerful enough instead of a too powerful here.

[pinkisemils]

Reviewable status: complete! all files reviewed, all discussions resolved

[raksooo]

Reviewed 3 of 6 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved