RoundCube - Security updates 1.6.13 and 1.5.13 released

[ccto]

Security fixes
- Fix CSS injection vulnerability reported by CERT Polska.
- Fix remote image blocking bypass via SVG content reported by nullcathedral.

ref.: https://roundcube.net/news/2026/02/08/security-updates-1.6.13-and-1.5.13