v0.7.1

Release post: https://mau.fi/blog/2024-09-mautrix-release/

• Updated to libsignal v0.57.1.
• Dropped support for unauthenticated media on Matrix.
• Added support for Matrix->Signal power level bridging (thanks to @maltee1 in #531).
• Changed voice message conversion to convert to aac instead of m4a, because Signal iOS doesn't appear to like ffmpeg's m4a files.
• Fixed outgoing sync messages not including disappearing start timestamp, which would cause native clients to disappear messages at the wrong time.
• Re-added notices about decryption errors.