Update default value to be blobless for checkout method key

[anthony-j-castro]

Description

This is a followup to #9611. We began rolling out blobless checkout as the default checkout method on November 3 (changelog: https://circleci.com/changelog/default-method-used-to-checkout-code-from-your-repository-is-changing-on-nov/). This updates the docs to reflect the new default.

Content Checklist

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Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs):

• Break up walls of text by adding paragraph breaks.
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