Replace outdated unmaintained dependencies with modern alternatives

[Copilot]

Overview

This PR addresses security and maintenance concerns by replacing two core dependencies that are no longer maintained with actively supported alternatives.

Changes

1. Markdown Rendering: blackfriday → goldmark

Replaced: github.com/russross/blackfriday/v2 v2.1.0 (unmaintained since 2020)
With: github.com/yuin/goldmark v1.7.13 (actively maintained)

The goldmark library is configured with GitHub-flavored markdown extensions to maintain full compatibility with existing functionality:

• Table support with HTML align attributes for backward compatibility
• Linkify for automatic URL linking
• Strikethrough text support
• Task list support

2. Email Sending: go-gomail → wneessen/go-mail

Replaced: github.com/go-gomail/gomail (unmaintained since 2016)
With: github.com/wneessen/go-mail v0.7.2 (actively maintained)

Updated the examples to use the modern go-mail API while preserving all existing functionality including sender formatting, multi-part messages (text/plain and text/html), and SMTP authentication.

Benefits

• Security: Both new libraries are actively maintained and receive regular security updates
• Performance: goldmark offers better performance than blackfriday
• Standards Compliance: wneessen/go-mail is more compliant with modern SMTP standards
• Future-proof: Active communities ensure continued support and bug fixes

Testing

• ✅ All existing tests pass without modification (100% success rate)
• ✅ Examples build and run successfully
• ✅ Email HTML/text generation verified
• ✅ CodeQL security analysis: 0 alerts
• ✅ No breaking changes to public API

Migration Impact

For library users: No changes required. This is a drop-in replacement with full backward compatibility.

For users referencing the examples: If you've based your email sending code on the examples, you'll need to update your imports and use the new go-mail API. See examples/main.go for the updated implementation.

Fixes #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Core dependencies are outdated and unmaintained</issue_title>
<issue_description>Hey! 👋

First off, thanks for forking this and adding some cool new features!

I was looking into using the library and noticed a couple of its core dependencies pose a potential security risk:

1. gomail is unmaintained since 2016.

   • github.com/go-gomail/gomail v0.0.0-20160411212932-81ebce5c23df
   • A maintained alternative could be https://github.com/wneessen/go-mail.

2. The markdown renderer blackfriday is unmaintained since 2020.

   • github.com/russross/blackfriday/v2 v2.1.0
   • A maintained alternative could be https://github.com/yuin/goldmark.

Would you consider updating them to more current, maintained versions?

Thanks 🙏</issue_description>

<agent_instructions>Please solve the issue with the outdated libraries and use the suggested replacements.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@ChronosMasterOfAllTime Good catch! Will look into alternatives and update accordingly

Fixes #21

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