Set Java 21 as minimal version

[JohnNiang]

What type of PR is this?

/kind improvement
/area core
/milestone 2.21.x

What this PR does / why we need it:

This PR sets Java 21 as minimal version. After doing that, Halo can only run on Java 21 or newer version.

Fortunately, all plugins and themes won't be affected by the restriction.

https://docs.gradle.org/current/userguide/toolchains.html#sec:release-flag-toolchain

Which issue(s) this PR fixes:

Fixes #6867

Does this PR introduce a user-facing change?

[Action Required] 更新最低运行环境为 Java 21

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.44%. Comparing base (04c27db) to head (cc2acc2).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #7451   +/-   ##
=========================================
  Coverage     59.44%   59.44%           
  Complexity     4142     4142           
=========================================
  Files           701      701           
  Lines         24014    24014           
  Branches       1543     1543           
=========================================
  Hits          14274    14274           
  Misses         9078     9078           
  Partials        662      662           

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[ruibaby]

Is it possible to take advantage of Virtual Threads now?

[JohnNiang]

Is it possible to take advantage of Virtual Threads now?

Already done in #7261.

[ruibaby]

Shouldn’t this also be updated here?

halo/.github/actions/setup-env/action.yaml

Line 18 in 7eadf7a

default: "17"

[sonarqubecloud]

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 SonarQube Cloud

[ruibaby]

/lgtm

需要对应修改 https://github.com/halo-dev/docs 中开发文档的部分。

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ruibaby

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