deps: fix RUSTSEC-2026-0097 by updating jwt-simple to 0.12.15

Update jwt-simple from 0.12.14 to 0.12.15, which transitively updates
rand from 0.8.5 to 0.8.6, fixing the unsoundness vulnerability
RUSTSEC-2026-0097.

Note: This update also adjusts thiserror from 2.0.18 to 1.0.69 due to
dependency resolution with sequoia-openpgp. Both versions are actively
maintained; this is not a security downgrade.

Assisted-by: IBM Bob
Signed-off-by: stevenhorsman <[email protected]>

Author: stevenhorsman