Add changelog entry for version 4.82.1

[Copilot]

Changes proposed in this request
Adds version 4.82.1 changelog entry to the top of CHANGELOG.md, positioned before the 4.82.0 entry. The entry documents:

• Removal of experimental flag requirement from IAuthenticationOperation (Remove experimental flag requirement from IAuthenticationOperation #5699)
• Security warning addition to ICustomWebUi documentation (Add security warning to ICustomWebUi documentation #5704)
• Implicit mTLS transport support for client assertion delegates (Adds support for implicit mTLS (Mutual TLS) transport for client assertion delegates #5670)

Testing
Documentation-only change. No functional testing required.

Performance impact
None.

Documentation

• All relevant documentation is updated.

Original prompt

Create a changelog entry for version 4.82.1 release.

Add the following changelog entry to CHANGELOG.md:

4.82.1
======

### Bug Fixes

* Remove experimental flag requirement from IAuthenticationOperation [#5699](https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/pull/5699)
* Add security warning to ICustomWebUi documentation [#5704](https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/pull/5704)

### Changes

* Adds support for implicit mTLS (Mutual TLS) transport for client assertion delegates [#5670](https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/pull/5670)

This should be added at the top of the CHANGELOG.md file, before the 4.79.1 entry.

This pull request was created from Copilot chat.

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