[v25.3.x] build/deps: upgrade libxml2 to v2.15.2 (CVE-2026-0990)

[tyson-redpanda]

Backport of PR #29788

[vbotbuildovich]

Retry command for Build#81703

please wait until all jobs are finished before running the slash command

/ci-repeat 1
skip-redpanda-build
skip-units
skip-rebase
tests/rptest/tests/write_caching_fi_e2e_test.py::WriteCachingFailureInjectionE2ETest.test_crash_all@{"use_transactions":false}

[vbotbuildovich]

CI test results

test results on build#81703

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
SimpleEndToEndTest	test_relaxed_acks	{"write_caching": false}	integration	https://buildkite.com/redpanda/redpanda/builds/81703#019ce37f-66a3-434a-97e7-dd4a616142fb	FLAKY	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0000, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.1000, p1=0.3487, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=SimpleEndToEndTest&test_method=test_relaxed_acks
WriteCachingFailureInjectionE2ETest	test_crash_all	{"use_transactions": false}	integration	https://buildkite.com/redpanda/redpanda/builds/81703#019ce37f-669f-40b3-8f58-4fa09d24ffd5	FLAKY	22/31	Test FAILS after retries.Significant increase in flaky rate(baseline=0.0943, p0=0.0055, reject_threshold=0.0100)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=WriteCachingFailureInjectionE2ETest&test_method=test_crash_all

[dotnwat]

restarted ci. there was one timeout in a storage test. i think you could safely force merge this.