chore: Rename `WebIdeConfig.extensionsGallerySettings`

Issue: Standardize extensions marketplace terminology (gitlab#512642 - closed)

• Part 1: chore: Rename some ExtensionsGallery to Extensi... (!424 - merged)
• Part 2: chore: Rename `WebIdeConfig.extensionsGallerySe... (!425 - merged)
• Part 3: chore: Finish renaming "gallery" to "marketplace" (!426 - merged)

What does this MR do and why?

• This introduces a extensionMarketplaceSetting to replace extensionsGallerySetting in the Web IDE config.
• It uses the approach used for handling deprecated baseUrl property to temporarily support both the new and old field.
• This supports this issue to standardize the ExtensionsMarketplace naming in the code to match the feature name.