[DOC] Update Linear Regression default optimizer

[rithin-pullela-aws]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request.
The default optimizer for linear regression has been updated from SIMPLE_SGD to ADA_GRAD.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
2.19 and above

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
This is the documentation page to be updated.
This is the PR corresponding to the update.
This is the issue.

[rithin-pullela-aws]

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