Re-enable CLN keysend interop test once a fixed CLN Docker image is available

[febyeji]

test_keysend in tests/integration_tests_cln.rs is currently marked #[ignore] via interop_scenario_test!.

CLN ≤v25.12.x hardcoded keysend min_final_cltv_expiry = 22, below LDK's minimum of 42, so peer→LDK keysend HTLCs were rejected. Fixed upstream in ElementsProject/lightning#9034 and released in CLN v26.04. Interop CI pins elementsproject/lightningd:v25.12.1, but no v26.04 image has been published to Docker Hub yet.

Unblock: bump the pinned CLN Docker image once v26.04 (or later) is published, then drop #[ignore] on test_keysend.

Context: PR #839 review thread.

[tnull]

Note that https://github.com/ElementsProject/lightning/releases/tag/v26.04 was just released (also enabling splicing by default), so hopefully the docker image will soon be available, too.