fix: use flex to correct avatar child position

[lcgash]

[English Template / 英文模板]

🤔 这个变动的性质是？

• 新特性提交
• 日常 bug 修复
• 站点、文档改进
• 演示代码改进
• [ ✔️] 组件样式/交互改进
• TypeScript 定义更新
• 包体积优化
• 性能优化
• 功能增强
• 国际化改进
• 重构
• 代码风格优化
• 测试用例
• 分支合并
• 工作流程
• 其他改动（是关于什么的改动？）

🔗 相关 Issue

fix: #47232

💡 需求背景和解决方案

📝 更新日志

修复高度过小时，avatar 组件内容因为 border 存在无法居中的问题

语言	更新描述
🇺🇸 英文	Fix the problem that the content of the avatar component is not centered when the height is less than 16px
🇨🇳 中文	修复 avatar 组件当高度小于 16px 内容不居中的问题

☑️ 请求合并前的自查清单

⚠️ 请自检并全部勾选全部选项。⚠️

• [ ✔️] 文档已补充或无须补充
• [✔️ ] 代码演示已提供或无须提供
• TypeScript 定义已补充或无须补充
• Changelog 已提供或无须提供

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[github-actions]

[github-actions]

Visual Regression Report for PR #47236 Failed ❌

Target branch: master (ebd4c15)
View Full Report

---

image name	expected	actual	diff
avatar-badge.compact.png
avatar-badge.compact.css-var.png
avatar-badge.dark.png
avatar-badge.dark.css-var.png
avatar-badge.default.png
avatar-badge.default.css-var.png
avatar-basic.compact.png
avatar-basic.compact.css-var.png
avatar-basic.dark.png
avatar-basic.dark.css-var.png

Check Full Report for details

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit ef3c1b8:

Sandbox	Source
antd reproduction template (forked)	Configuration

[MadCcc]

测试再看一下，snapshot 需要更新，效果看起来没什么问题

[lcgash]

测试再看一下，snapshot 需要更新，效果看起来没什么问题
snapshot 已经更新了，测试看起来没啥问题

[afc163]

snapshot 测试还是挂的。

[lcgash]

snapshot 测试还是挂的。

抱歉，没看见那个

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (441ba5f) 100.00% compared to head (ef3c1b8) 100.00%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master    #47236   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          732       732           
  Lines        12566     12563    -3     
  Branches      3298      3296    -2     
=========================================
- Hits         12566     12563    -3     

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[github-actions]

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