Improve notice about discontinued maintenance

[xSeagullx]

Context

Improve notice's wording, to clarify that code is still under Apache 2.0 license, but is no longer maintained here.

Point to the new closed source location for Wise employees to simplify transition and avoid broken links.

[platon-github-app-production]

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[Copilot]

Pull request overview

This PR improves the wording of the maintenance discontinuation notice in the README to clarify that while the repository is no longer actively maintained, the code remains available under the Apache 2.0 license. The changes address potential confusion about the open-source status and provide clearer guidance for Wise employees on where to find the actively maintained internal version.

• Clarifies that the repository is "no longer maintained" rather than "no longer open source"
• Explicitly states the code remains available under Apache 2.0 license terms
• Adds specific guidance for Wise employees to access the private repository

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[Copilot]

[nitpick] The phrase "The codebase has moved to a private repository accessible to authorized Wise personnel" is redundant with line 5 which states "We have moved the codebase to our internal systems". Consider consolidating or removing the redundant information to improve clarity and conciseness.