[dotnet] Add future-proofing note on Base64UrlEncoder

[RenderMichael]

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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.

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PR Type

documentation

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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.

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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

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[qodo-code-review]

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

🧪 No relevant tests

🔒 No security concerns identified

⚡ No major issues detected

[RenderMichael]

Should this type be internal?

[qodo-code-review]

PR Code Suggestions ✨

No code suggestions found for the PR.

[nvborisenko]

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