feat: deprecate mixpanel apiSecret

[Gauravudia]

What are the changes introduced in this PR?

Deprecating the API secret support, As this setting is not required anymore for authentication.
Related : rudderlabs/rudder-transformer#2833

What is the related Linear task?

Resolves INT-652

Please explain the objectives of your changes below

Put down any required details on the broader aspect of your changes. If there are any dependent changes, mandatorily mention them here

Any changes to existing capabilities/behaviour, mention the reason & what are the changes ?

N/A

Any new dependencies introduced with this change?

N/A

Any new checks got introduced or modified in test suites. Please explain the changes.

N/A

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Developer checklist

• My code follows the style guidelines of this project

• No breaking changes are being introduced.

• All related docs linked with the PR?

• All changes manually tested?

• Any documentation changes needed with this change?

• I have executed schemaGenerator tests and updated schema if needed

• Are sensitive fields marked as secret in definition config?

• Is the PR limited to 10 file changes?

• Is the PR limited to one linear task?

Reviewer checklist

• Is the type of change in the PR title appropriate as per the changes?

• Verified that there are no credentials or confidential data exposed with the changes.

[linear]

INT-652 Mixpanel Cloud Mode: Deprecate the support for /track endpoint

Reviewers: sudip , shrouti

RudderStack is supporting mixpanel /track and /import endpoint for sending the events.

Mixpanel suggest to use /import endpoint for server side integration, As it is more scalable as compared to /track endpoint.

	/track	/import
Events per request	50	2000
Authentication	Project Token, intended for untrusted clients.	Project Secret, intended for server-side integration.Update: Project Token is also supported now
Compression	None	Gzip allowed
Content-Type	application/x-www-form-urlencoded	application/json or application/x-ndjson
Ingesting historical events	Last 5 days only.	Any time after 1971-01-01.

Both track and import APIs are synchronous in nature. It update the data in mixpanel in realtime.

References of the requirement (Slack/Notion/PRD doc)

Not a customer requirement, Marking this a new feature to make the integration more reliable, increase batching efficiency.

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• If there are any design-related discussions or changes that require a design review, please link the N/A here.

• Design/Research doc: N/A

• Test cases doc: https://www.notion.so/rudderstacks/Test-Results-a5d9914fca554cba8b373b81d10549d9 (Refer template here)

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• Changes tested locally as well as in dev/staging environments.
• Existing test cases updated for the new feature.
• Has the task been checked against the design request questionnaire - https://www.notion.so/rudderstacks/When-to-ask-for-a-design-review-a3534d5e4f5f4fcb989847ca9cd96189

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• The Slack link for the respective thread is attached.
• Docs card attached.
• GitHub PR is attached to the task.
• Release label attached to the task.
• The project associated with the task is linked, if any.
• Document the monitoring steps here

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e82967f) to head (d3871a9).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop     #1304   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines           53        53           
  Branches         7         7           
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  Hits            53        53           

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