Release 3.1.1

[donnchawp]

Prepares WP Super Cache 3.1.1 for release.

• Bumps plugin version to 3.1.1 (readme.txt Stable tag + wp-cache.php header).
• Bumps Tested up to to 7.0.
• Adds the 3.1.1 changelog section.

Changelog

• Security: harden supercache filename generation so request-derived data cannot escape the cache directory. (Restrict supercache filename to a safe character set #1050)
• Fix: avoid a fatal error on PHP 8+ when closing an unopened file handle in supercache-only mode. (Guard fclose() against false stream in supercache-only mode #1053)
• Fix: cast the blog ID in wp_cache_clear_cache() so non-integer callers no longer trigger a multisite-only function on single-site installs. (Cast $blog_id to int in wp_cache_clear_cache() #1054)
• Fix: no longer append HTML debug comments to REST API, Ajax, JSON, WooCommerce API, and XML-RPC responses. (Fix: hide debug HTML comments in REST and Ajax responses #1021)
• Fix: avoid a PHP warning when a comment has already been deleted before the cache-clear routine runs. (Fix: add null check for comment in wp_cache_get_postid_from_comment() #1043)

WPSC_VERSION_ID (1.12.1) is intentionally left unchanged — it tracks a separate internal lineage, not the release version.