Update documentation for v5.0 release

[Copilot]

This PR updates the documentation to reflect the upcoming v5.0 release by updating version references throughout the documentation from v4.x to v5.x.

Changes

README.md

• Updated the migration section to reference "Migrating from v4.x to main" (previously v3.x to main)
• Updated the migration guide link to point to "Migration Guide for v5.x" (previously v4.x)
• Updated the source version reference to indicate upgrading from the v4.x branch

doc/migration.md

• Updated the document title from "Migration Guide for v4.x" to "Migration Guide for v5.x"
• Updated all version references throughout the document from v4.x to v5.x
• The guide now describes how to migrate from v4.x to v5.x (previously from v3.x to v4.x)

Verification

All markdown documentation files have been checked for version references:

• ✅ All doc/*.md files reviewed
• ✅ SECURITY.md checked (no version references)
• ✅ samples/proxy/README.md checked (no version references)

The documentation is now consistent and ready for the v5.0 release.

Fixes: Update docs for v5.0 in next branch

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update docs for v5.0 in next branch</issue_title> <issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes microsoft/cppgraphqlgen#330

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