refactor: remove trailing slash in site url for notification

[guqing]

What type of PR is this?

/kind improvement /area core /milestone 2.20.x

What this PR does / why we need it:

修复 external-url 配置带了尾部斜杠导致邮件通知的查看通知链接无法访问的问题

Which issue(s) this PR fixes:

Fixes #6655

Does this PR introduce a user-facing change?

修复 external-url 配置带了尾部斜杠导致邮件通知的查看通知链接无法访问的问题

[sonarcloud[bot]]

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.13%. Comparing base (6cd8dc8) to head (b4bab2b). Report is 72 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6660 +/- ## ============================================ - Coverage 58.18% 58.13% -0.06% - Complexity 3774 3928 +154 ============================================ Files 651 674 +23 Lines 22125 23068 +943 Branches 1538 1574 +36 ============================================ + Hits 12873 13410 +537 - Misses 8641 9040 +399 - Partials 611 618 +7 ```

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[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang

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