Remove Legacy Landing Pages from Settings Dropdowns

[n7studios]

Summary

Removes legacy Kit landing pages from every settings dropdown across the plugin, while preserving any legacy landing page (by ID or by pre-1.9.6 URL string) already saved to a page's post meta.

Testing

Tests added:

Integration (tests/Integration/ResourceLandingPagesTest.php):

• testGetNonLegacy — get_non_legacy() returns v4 landing pages and excludes legacy; every returned page has embed_url and no url key
• testNonLegacyExist — helper returns true when v4 pages exist
• testIsLegacyWithLegacyID — true for legacy ID
• testIsLegacyWithURLString — true when passed a URL string (pre-1.9.6 storage)
• testIsLegacyWithV4ID — false for v4 ID
• testIsLegacyWithInvalidID — false for an ID that doesn't resolve

End-to-end:

• ClassicEditorLandingPageCest::testLegacyLandingPagesExcludedFromMetaboxDropdown — Classic editor metabox Landing Page dropdown has no legacy option
• ClassicEditorLandingPageCest::testMetaboxPreservesSelectedLegacyLandingPageByID — page preseeded with legacy ID in post meta shows as selected in metabox AND still renders on the frontend, including the WordPress site icon replacing the Kit favicon
• ClassicEditorLandingPageCest::testMetaboxPreservesSelectedLegacyLandingPageByURL — page preseeded with a legacy URL string (pre-1.9.6 storage) shows as selected in metabox
• BlockEditorLandingPageCest::testLegacyLandingPagesExcludedFromPluginSidebar — Gutenberg plugin sidebar Landing Page dropdown has no legacy option
• BlockEditorLandingPageCest::testPluginSidebarPreservesSelectedLegacyLandingPage — page preseeded with legacy ID shows as selected in Gutenberg plugin sidebar
• PageLandingPageSetupWizardCest::testLegacyLandingPagesExcludedFromSetupWizard — Setup Wizard landing page picker has no legacy option

Tests removed (all previously selected a legacy landing page from a dropdown that no longer offers it):

• ClassicEditorLandingPageCest::testAddNewPageUsingDefinedLegacyLandingPage
• ClassicEditorLandingPageCest::testLegacyLandingPageSiteIcon (site-icon coverage moved into testMetaboxPreservesSelectedLegacyLandingPageByID)
• BlockEditorLandingPageCest::testAddNewPageUsingDefinedLegacyLandingPage
• BlockEditorLandingPageCest::testLegacyLandingPageSiteIcon
• PageLandingPageSetupWizardCest::testAddNewLegacyLandingPage

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)