fix: resolve button height overflow in signup form on older Safari

[ruibaby]

What type of PR is this?

/area core
/kind bug
/milestone 2.20.x

What this PR does / why we need it:

修复在低版本 Safari 浏览器中，注册表单中的按钮高度溢出的问题。

Which issue(s) this PR fixes:

Fixes #6910

Does this PR introduce a user-facing change?

修复在低版本 Safari 浏览器中，注册表单中的按钮高度溢出的问题。

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.22%. Comparing base (eff73dc) to head (197429f).
Report is 22 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6912      +/-   ##
============================================
+ Coverage     56.99%   57.22%   +0.22%     
- Complexity     3999     4005       +6     
============================================
  Files           714      712       -2     
  Lines         24110    24113       +3     
  Branches       1585     1592       +7     
============================================
+ Hits          13742    13799      +57     
+ Misses         9756     9699      -57     
- Partials        612      615       +3     

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[guqing]

/lgtm

[f2c-ci-robot]

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