docs(saml2): rename migration guide and update navigation entry

Renamed 'saml2-migration-guide.adoc' to 'saml-extension-migration.adoc' for clarity,
and updated the nav link text to 'Migrating from Spring Security SAML Extension'.

Signed-off-by: snowykte0426 <[email protected]>

Author: snowykte0426

docs/modules/ROOT/nav.adoc

@@ -89,7 +89,7 @@
 **** xref:servlet/saml2/login/authentication.adoc[SAML2 Authentication Responses]
 *** xref:servlet/saml2/logout.adoc[SAML2 Logout]
 *** xref:servlet/saml2/metadata.adoc[SAML2 Metadata]
-*** xref:saml2/saml2-migration-guide.adoc[SAML 2.0 Migration Guide]
+*** xref:servlet/saml2/saml-extension-migration.adoc[Migrating from Spring Security SAML Extension]
 ** xref:servlet/exploits/index.adoc[Protection Against Exploits]
 *** xref:servlet/exploits/csrf.adoc[]
 *** xref:servlet/exploits/headers.adoc[]