Update release notes for 0.5.15 release

[julientinguely-da]

part of https://github.com/DACH-NY/canton-network-internal/issues/3964

[static]

Pull Request Checklist

Cluster Testing

• If a cluster test is required, comment /cluster_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.
• If a hard-migration test is required (from the latest release), comment /hdm_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.

PR Guidelines

• Include any change that might be observable by our partners or affect their deployment in the release notes.
• Specify fixed issues with Fixes #n, and mention issues worked on using #n
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Merge Guidelines

• Make the git commit message look sensible when squash-merging on GitHub (most likely: just copy your PR description).

[martinflorian-da]

Thanks!

[moritzkiefer-da]

thanks