sysutils/terraform: Update to 1.15.9, fix LICENSE

Update from 1.15.3 to 1.15.9. 1.15.9 updates go-slug to v0.18.3,
addressing CVE-2026-14978, an exclusion bypass in .terraformignore
handling. The version proposed in the PR, 1.15.8, still carries the
affected go-slug v0.18.1, hence the jump to 1.15.9.

The remaining changes are bugfixes: crashes in init and console,
wrong exit codes for variable errors in plan, query and refresh,
child module and submodule validation, several module installation
edge cases, and concurrency safety in the config parsers.

PR: 297296
Security: CVE-2026-14978
Approved by: maintainer timeout (dutra)
Sponsored by: Netzkommune GmbH

(cherry picked from commit 470005eb0237802792ed6d527374f0f1301e1967)