Update WP compatibility check in gutenberg_pre_init() (#29938)

* Update `gutenberg_pre_init()` compat check to WP 5.6

The plugin relies on `WP_Block_Supports`, which was added in 5.6
and `register_block_format()`, which was added in 5.5.

* Do not bail early after adding WP version notice

When `return` is used here, Gutenberg is not loaded and the other
hooks added in this file use functions that are only loaded with
Gutenberg.

* Update gutenberg.php

* Update gutenberg.php

Co-authored-by: Greg Ziółkowski <grzegorz@gziolo.pl>

gutenberg.php

@@ -26,7 +26,7 @@
 function gutenberg_wordpress_version_notice() {
 	echo '<div class="error"><p>';
 	/* translators: %s: Minimum required version */
-	printf( __( 'Gutenberg requires WordPress %s or later to function properly. Please upgrade WordPress before activating Gutenberg.', 'gutenberg' ), '5.3.0' );
+	printf( __( 'Gutenberg requires WordPress %s or later to function properly. Please upgrade WordPress before activating Gutenberg.', 'gutenberg' ), '5.6' );
 	echo '</p></div>';
 
 	deactivate_plugins( array( 'gutenberg/gutenberg.php' ) );
@@ -61,7 +61,10 @@ function gutenberg_pre_init() {
 	// Strip '-src' from the version string. Messes up version_compare().
 	$version = str_replace( '-src', '', $wp_version );
 
-	if ( version_compare( $version, '5.3.0', '<' ) ) {
+	// Compare against major release versions (X.Y) rather than minor (X.Y.Z)
+	// unless a minor release is the actual minimum requirement. WordPress reports
+	// X.Y for its major releases.
+	if ( version_compare( $version, '5.6', '<' ) ) {
 		add_action( 'admin_notices', 'gutenberg_wordpress_version_notice' );
 		return;
 	}