Make BlockUI compatible with jQuery 4.x

[DanielJudele]

This pull request updates BlockUI 2.70 to work correctly with jQuery 4.x by removing legacy browser code paths and internal APIs that were removed from jQuery core. The public API, defaults, and runtime behavior of BlockUI remain unchanged.
What changed

• Replaced $.isFunction with typeof x === "function".
• Replaced .bind with .on
• Replaced .unbind with .off
• Removed unused variables
• Format the code

What did not change

• No functional or behavioral changes for supported browsers
• No breaking changes to API, options, or callbacks
• No dependency changes other than ensuring compatibility with jQuery 4.x
• No changes to the BlockUI public API ($.blockUI, $.unblockUI, .block(), .unblock())

Motivation
While BlockUI continues to work with jQuery ≤ 3.x, some of its legacy detection logic is no longer applicable or reachable under jQuery 4.
This change ensures:

• Forward compatibility with jQuery 4.x
• Alignment with jQuery’s current browser support policy

Testing
Verified with jQuery 4.x in modern browsers (Chrome, Edge)
Verified that existing BlockUI usage patterns continue to work as expected