build/deps: upgrade libxml2 to v2.15.3 (CVE-2026-6732)

[tyson-redpanda]

Upgrades libxml2 from 2.15.2 to 2.15.3 to fix CVE-2026-6732 (type confusion vulnerability in XSD validation with internal entity references, CVSS 7.1 High). Also affects v25.3.x (separate PR) and dev (#30392).

This PR depends on redpanda-data/vtools#4233 being merged first so the artifact is available in S3.

Backports Required

• none - not a bug fix
• none - this is a backport

Release Notes

Bug Fixes

• Upgraded libxml2 to v2.15.3 to fix CVE-2026-6732 type confusion vulnerability in XSD validation.

FIXES=CORE-16201

[vbotbuildovich]

CI test results

test results on build#84161

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(PASS)	ShadowLinkingReplicationTests	test_auto_prefix_trimming	{"source_cluster_spec": {"cluster_type": "redpanda"}, "with_failures": true}	integration	https://buildkite.com/redpanda/redpanda/builds/84161#019e0291-3cf8-4c27-bdc2-72befbb81a41	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0005, 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=ShadowLinkingReplicationTests&test_method=test_auto_prefix_trimming
FLAKY(PASS)	WriteCachingFailureInjectionE2ETest	test_crash_all	{"use_transactions": false}	integration	https://buildkite.com/redpanda/redpanda/builds/84161#019e0291-3cfd-40fd-9143-20112797b29e	10/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.1031, p0=1.0000, reject_threshold=0.0100. adj_baseline=0.2785, p1=0.0382, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=WriteCachingFailureInjectionE2ETest&test_method=test_crash_all