rpk: rename decommission --force flag

[r-vasquez]

To skip-liveness-check. The --force flag is not clear because it implies that it modifies the behavior of the decommissioning, but it just skips checks.

Please note that I'm not hiding the flag anymore; it is advertised in our help text and in every error message, so it's better to have it present, showing what it does in the help text.

Fixes INC-1056

Example:

Using --force will throw a warning, but still work

$ rpk redpanda admin brokers decommission 0 --force
Flag --force has been deprecated, use --skip-liveness-check
Success, broker 0 decommission started.  Use 'rpk redpanda admin brokers decommission-status 0' to monitor data movement.

With the new flag

$ rpk redpanda admin brokers decommission 0 --skip-liveness-check
Success, broker 0 decommission started.  Use 'rpk redpanda admin brokers decommission-status 0' to monitor data movement.

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
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

Improvements

• rpk redpanda admin brokers decommission: --force flag has been deprecated and renamed to --skip-liveness-check.

[Copilot]

Pull request overview

This PR renames the --force flag to --skip-liveness-check in the rpk redpanda admin brokers decommission command to better clarify its purpose. The flag skips liveness checks rather than forcing decommissioning behavior, and the new name makes this explicit.

Changes:

• Renamed the force variable to skipLivenessCheck throughout the decommission command implementation
• Updated all error messages and help text to reference --skip-liveness-check instead of --force
• Deprecated the old --force flag while maintaining backward compatibility by binding both flags to the same variable
• Fixed a minor formatting issue in the success message (backtick placement)

[vbotbuildovich]

CI test results

test results on build#79515

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
WriteCachingFailureInjectionE2ETest	test_crash_all	{"use_transactions": false}	integration	https://buildkite.com/redpanda/redpanda/builds/79515#019be78b-35bb-4633-876b-876de8e561c2	FLAKY	9/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.0818, p0=0.5742, reject_threshold=0.0100. adj_baseline=0.2260, p1=0.3026, 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