Update README's Logfile collection section, add deprecation notice

[chrisberkhout]

Proposed commit message

Update README's Logfile collection section, add deprecation notice Adds a deprecation notice to the top of the README explaining the unavailability of the legacy Message Trace API for new tenants. There are two changes in the Logfile collection section: - Clarify the authentication guidance that was added when basic authentication was deprecated. The script already doesn't use basic authentication. It's still helpful to link to the documentation of the various authentication methods. - Update the example PowerShell script for the V2 cmdlet, by changing the cmdlet name and the pagination logic. Links related to the V2 migration: - ExchangePowerShell / Get-MessageTraceV2 cmdlet documentation https://learn.microsoft.com/en-us/powershell/module/exchangepowershell/get-messagetracev2?view=exchange-ps - Announcing General Availability (GA) of the New Message Trace in Exchange Online (2025-06-03) https://techcommunity.microsoft.com/blog/exchange/announcing-general-availability-ga-of-the-new-message-trace-in-exchange-online/4420243) - Announcing Public Preview of the New Message Trace in Exchange Online (2024-12-16) https://techcommunity.microsoft.com/blog/exchange/announcing-public-preview-of-the-new-message-trace-in-exchange-online/4356561 

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

Related issues

• Relates [microsoft_exchange_online_message_trace]: MS will deprecate the Exchange Online Message Trace API #14349

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cc @chrisberkhout

[StacieClark-Elastic]

LGTM