bazel: Update to v9

[StephanDollberg]

• Update to v9
• Set --experimental_remote_repo_contents_cache

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v26.1.x
• v25.3.x
• v25.2.x

Release Notes

• none

[Copilot]

Pull request overview

Updates the repository’s Bazel toolchain/configuration to Bazel v9 and aligns module dependencies and build settings to accommodate Bazel 9 behavior changes.

Changes:

• Bump Bazel to 9.0.0 and add Bazel 9 compatibility flags in .bazelrc.
• Update several Bzlmod dependencies and apply a zlib override/patch to expose unprefixed public headers.
• Adjust the Iceberg REST catalog jar genrule to set a JDK/JAVA_HOME before invoking Gradle.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/java/iceberg-rest-catalog/BUILD	Sets JAVA_HOME/PATH for the Gradle genrule that builds the test jar.
bazel/thirdparty/zlib-public-unprefixed-headers.patch	Patches zlib’s BUILD to include unprefixed headers in hdrs.
MODULE.bazel	Bumps module versions and adds a single_version_override to apply the zlib patch.
.bazelversion	Updates Bazel version to 9.0.0.
.bazelrc	Enables remote repo contents caching and adds Bazel 9-related build compatibility options.

[vbotbuildovich]

CI test results

test results on build#83881

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FAIL	RpkDebugBundleTest	test_remote_debug_bundle_default	null	integration	https://buildkite.com/redpanda/redpanda/builds/83881#019dde5c-578f-4cab-b66d-f5a399805d3e	0/1		https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=RpkDebugBundleTest&test_method=test_remote_debug_bundle_default