Make ApplicationContext inaccessible in ITemplateContext

[JohnNiang]

What type of PR is this?

/kind improvement /area core /area plugin /milestone 2.20.x

What this PR does / why we need it:

This PR disables access to ApplicationContext using ITemplateContext.

Does this PR introduce a user-facing change?

None

[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

Attention: Patch coverage is 33.33333% with 28 lines in your changes missing coverage. Please review.

Project coverage is 58.04%. Comparing base (6cd8dc8) to head (849335e). Report is 79 commits behind head on main.

Files with missing lines	Patch %	Lines
.../halo/app/theme/dialect/SecureTemplateContext.java	24.32%	25 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6680 +/- ## ============================================ - Coverage 58.18% 58.04% -0.14% - Complexity 3774 3942 +168 ============================================ Files 651 676 +25 Lines 22125 23154 +1029 Branches 1538 1579 +41 ============================================ + Hits 12873 13440 +567 - Misses 8641 9090 +449 - Partials 611 624 +13 ```

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[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing

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[guqing]

/kind api-change 此 PR 禁止插件通过 SpringContextUtils 获取 ApplicationContext，如果插件使用到此方法则需要使用依赖注入来代替