feat: rename aiAssisted configuration namespace to duo

Description

Renames all settings in gitlab.aiAssistedCodeSuggestions.* to gitlab.duoCodeSuggestions.*. Updates all docs to reflect the new naming.

The following items are handled: openTabsContext, enabled, preferredAccount, enabledSupportedLanguages, additionalLanguages.

Due to these old config items being effectively removed, I've marked this MR as a breaking change.

To mitigate impact on customer, there is a new config migration step when the extension activates, which ports old config to the new naming. So no one should need to notice this change.

The migration has roughly this logic:

- if no old value found, exit early, otherwise:
  - if new value is not found, migrate old value to new value (if the value is valid)
  - if a new value already exists, don't replace it with old value
  - remove old value.

It deals with the different configuration targets (user scoped settings, workspace scoped settings etc) to ensure values are migrate from/to the same place.

Related MRs:

A followup MR will update references in the code to add consistency with the new naming: refactor: rename aiAssisted configuration names... (!1853 - merged)

Gitlab repo docs update: !1847 (merged)

GitLab WebUI update to be included when adding updated VSCode extension into the WebIDE: gitlab-web-ide!362 (closed)

Related Issues

Resolves #1459 (closed)

How has this been tested?

• Start extension host, and open your test project
• ctrl+shift+p to
  • Open user settings (JSON)
  • Open workspace settings (JSON)
• Ensure each config file has some settings with the old aiAssistedCodeSuggestions namespace
• Close and re-open the extension host
• Verify settings have migrated to new namespace
• Check Gitlab Workflow output logs for config migration entries

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes

Closes #1459 (closed)