integrations: Remove deprecated !avatar syntax from git integration

[taylornlane]

Fixes: #632

The !avatar() markdown syntax was removed from Zulip some time ago, but was still being used in the git integration. This commit replaces all remaining uses with plain text formatting.

Changes:

• zulip/integrations/git/post-receive: updated default fallback commit format
• zulip/integrations/git/zulip_git_config.py: updated the active
  format_commit_message() function and the commented example

How did you test this PR?
Ran the existing test suite (pytest zulip_bots/) and confirmed all 42 tests pass. Verified no remaining !avatar references exist in source files using grep.

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).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.