CLSA-2026-1778297730 exim: Fix of 2 CVEs

CVE-2026-40685: dewrap OOB read/write on trailing backslash in JSON header - CVE-2026-40687: SPA authenticator OOB read/write and base64 decode infoleak - Refresh Exim-Maintainers-Keyring.asc to verify the 4.99.x release tarball signature...