Remove Extension Marketplace feature flags

Issue [Feature flag] Rollout of `web_ide_extensions_m... (#459028 - closed)

What does this MR do and why?

It removes the web_ide_extensions_marketplace and the vscode_extension_marketplace_settings feature flags together. These feature flags were enabled by default on gitlab.com and self-managed instances on 17.11. The feature flag removal involves the following changes:

1. Remove all the references to these feature flags in the codebase.
2. Update the user documentation to reflect the removal of these feature flags.
3. Turn the migration db/migrate/20250228183319_migrate_vscode_extension_marketplace_feature_flag_to_data.rb into a no-op. This was a data-migration that created an Instance-Level Setting that enables the Extension Marketplace if they had opted to enable the web_ide_extensions_marketplace feature flag before it was enabled by default.

References

Screenshots or screen recordings

Screenshot	Description
	User Settings - GitLab Self-managed licensed (Workspaces Enabled)
	User Settings- FOSS Only (No Workspaces)
	User Settings - GitLab Self-managed licensed (Workspaces Enabled) when Extension Marketplace is disabled at the instance level
	User Settings - FOSS Only (No Workspaces) when the Extension Marketplace is disabled at the instance level
	Instance-level Settings - GitLab Self-managed licensed (Workspaces Enabled)
	Instance-level Settings - FOSS Only (No Workspaces)
	Extension Marketplace disabled at the user level
	Extension Marketplace disabled at the instance level

How to set up and validate locally

1. In the user preferences page in your GDK (https://gdk.test:3443/-/profile/preferences), check that the integrations section displays the Extension Marketplace settings for the following scenarios:
   1. Extension Marketplace enabled in the admin settings.
   2. Extension Marketplace disabled in the admin settings.
   3. FOSS_ONLY mode.
2. In the Admin General Settings, check that Extension Marketplace settings (https://gdk.test:3443/admin/application_settings/general#js-extension-marketplace-settings) appear in the following scenarios:
   1. FOSS_ONLY mode.
   2. Licensed instance.

MR acceptance checklist

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