fix!: remove deprecated secret keys and secret autogeneration for camunda 8.9

[bkenez]

Which problem does the PR fix?

Closes #4839
Closes #5104

What's in this PR?

Removes all deprecated secret configuration keys from chart 8.9 that were deprecated in 8.8 with the new secret standard (secret.existingSecret/existingSecretKey/inlineSecret). Also removes autogenerated secret functionality (global.secrets.autoGenerated and the secret-camunda.yaml template), eliminating the need for users to provide initial passwords and improving the default security posture.

values.yaml & schema:

• Remove 77 lines of deprecated keys from values.yaml across 14 config sections (identity auth, connectors, orchestration, optimize, web-modeler, elasticsearch, keycloak, postgresql)
• Remove global.secrets configuration block (autoGenerated, name, annotations)
• Remove corresponding entries from values.schema.json

Deleted legacy templates:

• templates/common/secret-camunda.yaml — autogenerated secret template (removed, users must provide credentials)
• identity/postgresql-secret.yaml — legacy Identity PostgreSQL secret
• web-modeler/secret-restapi.yaml — legacy Web Modeler REST API secret

Template cleanup (14 files):

• Remove shouldAutogenerateSecret helper function and simplify normalizeSecretConfiguration
• Remove legacy fallback paths from _helpers.tpl and deployment templates
• Remove plaintextKey/legacyKeyField params from secret config helper calls
• Add 42 keyRemoved constraints in constraints.tpl for deprecated keys and autoGenerated settings with actionable migration messages

Integration test migration (15 files):

• Migrate all integration test values files from deprecated existingSecret.name (object form) and top-level existingSecret/existingSecretKey to the new secret.existingSecret + secret.existingSecretKey format
• Remove autoGenerated: false settings from test values
• Add missing existingSecretKey values alongside every existingSecret reference
• Convert commented-out sections to new format

Unit tests:

• Remove autogenerated secret test files (secret_test.go, helpers_autogenerate_secret_test.go, goldenfiles_test.go, secret-camunda.golden.yaml)
• Remove tests for deleted templates
• Update secret-related test assertions for the new format
• Regenerate golden files

Checklist

Before opening the PR:

• In the repo's root dir, run make go.update-golden-only.
• There is no other open pull request for the same update/change.
• Tests for charts are added (if needed).

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• Did all checks/tests pass in the PR?