Fix WordPress 6.8.2 compatibility and PHP 8.0+ compatibility issues

[hariombalhara]

Fix WordPress 6.8.2 compatibility and PHP 8.0+ compatibility issues

Summary

This PR resolves critical compatibility issues preventing the Cal.com WordPress plugin from working with modern WordPress installations:

Primary Fix: Replaced str_contains() function (PHP 8.0+) with strpos() !== false for backward compatibility. The plugin was claiming PHP 5.2.4+ support but using PHP 8.0+ functions, causing fatal errors on many WordPress sites still running older PHP versions.

Version Updates: Updated WordPress compatibility from 6.0.1 to 6.8.2 (latest) and PHP requirement from unrealistic 5.2.4 to more practical 5.6.

Review & Testing Checklist for Human

• Test on PHP 5.6-7.4 - Install and activate plugin on older PHP versions where str_contains() doesn't exist (this was the original failure case)
• Test actual Cal.com embeds - Verify shortcodes work with real Cal.com booking URLs: [cal url="/username/meeting"]
• Test both embed modes - Verify inline (type="1") and popup (type="2") embeds render correctly
• Install on WordPress 6.8.2 - Confirm plugin installs and activates without errors on latest WordPress
• Verify logic equivalence - Double-check that strpos($string, $needle) !== false behaves identically to str_contains($string, $needle) in all edge cases

Recommended Test Plan: Set up WordPress on PHP 7.4, install plugin, create test page with [cal url="/test/30min" type="1"] and [cal url="/test/30min" type="2" text="Book Now"] shortcodes, verify both render without PHP errors.

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Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    A["cal.com.php<br/>(Main Plugin File)"]:::context
    B["inc/class.embed.php<br/>(Shortcode Handler)"]:::major-edit
    C["readme.txt<br/>(Plugin Metadata)"]:::minor-edit
    D["WordPress 6.8.2<br/>(Target Environment)"]:::context
    E["PHP 5.6-8.1<br/>(Compatibility Range)"]:::context

    A -->|includes| B
    B -->|str_contains to strpos fix| E
    C -->|updated requirements| D
    C -->|updated requirements| E

    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end

    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• Critical Fix: The str_contains() function was introduced in PHP 8.0, but plugin claimed PHP 5.2.4+ support
• Testing Performed: Verified plugin loads without fatal errors on PHP 8.1 and shortcode generates expected HTML
• Not Tested: Actual Cal.com embed functionality with live booking URLs - requires human verification
• Session Details: Requested by @hariombalhara, session: https://app.devin.ai/sessions/9de988b874ca446c9c1d5417152d51a6