Remove backwards-compatible authentication config

[ambient-code]

Summary

• Remove the legacy authentication configmap key and authenticationConfig Helm value that were kept for backwards compatibility, with TODO comments saying "remove in 0.7.0"
• Remove the now-unused oidc.LoadAuthenticationConfiguration function and its helper newJWTAuthenticator (the config package has its own equivalent implementations that remain in use)
• Update Dex documentation to reference spec.authentication.jwt on the Jumpstarter resource instead of the removed jumpstarter-controller.authenticationConfiguration Helm value

Test plan

• go build ./... passes in the controller module with no errors
• Verify Helm chart renders correctly without authenticationConfig value
• Verify existing deployments using the new config key continue to work

Closes #530

🤖 Generated with Claude Code

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• Commit unit tests in branch ambient-fix/530-remove-auth-compat

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[ambient-code]

CI Status Update

The pytest failure in jumpstarter/exporter/hooks_test.py::TestHookExecutor::test_exec_bash appears to be unrelated to this PR's changes.

This PR only modifies:

• Go controller code (config.go, oidc/config.go)
• Helm templates and values
• Documentation

The failing test is a macOS-specific Python test that validates bash substring syntax in hook execution. This is not touched by any of the authentication config removal changes.

The test expects the output BASH_OK: world from the bash command V="hello_world"; echo "BASH_OK: ${V:6:5}" but it's not appearing in the mock logger calls on macOS-15 with Python 3.11.

Is this a known flaky test on macOS, or should we investigate further?