rpk: set default language for app generate

[paulohtb6]

Set the default language for rpk generate app to go since it's the only one available today.

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
• v25.2.x
• v25.1.x
• v24.3.x

Release Notes

Improvements

• On rpk generate app, set go as the default language.

[Copilot]

Pull Request Overview

This PR sets a default language for the rpk generate app command to improve user experience. Previously, users had to explicitly specify a language, but now "go" is set as the default since it's currently the only supported language.

[vbotbuildovich]

CI test results

test results on build#76229

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
OffsetForLeaderEpochTest	test_offset_for_leader_epoch_transfer	null	integration	https://buildkite.com/redpanda/redpanda/builds/76229#019a7e4d-5c12-487a-92e8-f26c57878cca	FLAKY	20/21	upstream reliability is '100.0'. current run reliability is '95.23809523809523'. drift is 4.7619 and the allowed drift is set to 50. The test should PASS	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=OffsetForLeaderEpochTest&test_method=test_offset_for_leader_epoch_transfer
NodeWiseRecoveryTest	test_node_wise_recovery	{"dead_node_count": 1}	integration	https://buildkite.com/redpanda/redpanda/builds/76229#019a7e53-3a5b-4880-9d3d-d27824c58713	FLAKY	16/17		https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=NodeWiseRecoveryTest&test_method=test_node_wise_recovery