entrypoint: Add SOCIAL_AUTH_SYNC_ATTRS_DICT.

[envy]

SOCIAL_AUTH_SYNC_ATTRS_DICT is a dict; add it to the list of settings that are not strings. See https://github.com/zulip/zulip/blob/main/zproject/prod_settings_template.py#L519

Fixes:

How did you test this PR?

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
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Individual commits are ready for review (see commit discipline).

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Completed manual review and testing of the following:

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