[dotnet] Add future-proofing note on Base64UrlEncoder

[RenderMichael]

User description

Thanks for contributing to Selenium!
A PR well described will help maintainers to quickly review and merge it

Before submitting your PR, please check our contributing guidelines.
Avoid large PRs, help reviewers by making them as simple and short as possible.

Description

Addresses request in #14813

Motivation and Context

This note will help us remember about the Base64Url type that is available on .NET 9+

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

---

PR Type

documentation

---

Description

• Enabled nullable reference types in the Base64UrlEncoder class to improve code safety.
• Added a comment to note the potential use of the built-in Base64Url type available in .NET 9+, addressing issue [dotnet] [🚀 Feature]: Consider using the new Base64Url #14813.

---

Changes walkthrough 📝

	Relevant files
Documentation	Base64UrlEncoder.cs Add future-proofing note and enable nullable reference types dotnet/src/webdriver/Internal/Base64UrlEncoder.cs Enabled nullable reference types. Added a comment about future usage of Base64Url type in .NET 9+. +5/-0

---

💡 PR-Agent usage: Comment /help "your question" on any pull request to receive relevant information

[qodo-merge-pro]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ No major issues detected

[RenderMichael]

Should this type be internal?

[qodo-merge-pro]

PR Code Suggestions ✨

No code suggestions found for the PR.

[nvborisenko]

We kept origin, goal, context. Thank you Michael.