Use @POST for form field validation

[uhafner]

Downstream PR of jenkinsci/plugin-util-api-plugin#66.

Use @POST for field validation. For details see https://www.jenkins.io/doc/developer/security/form-validation/.

[codecov]

Codecov Report

Merging #702 (ee77bb9) into master (ea7f43d) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #702      +/-   ##
============================================
+ Coverage     80.72%   80.74%   +0.01%     
  Complexity     1502     1502              
============================================
  Files           240      240              
  Lines          5406     5405       -1     
  Branches        371      371              
============================================
  Hits           4364     4364              
+ Misses          886      885       -1     
  Partials        156      156              

Impacted Files	Coverage Δ	Complexity Δ
...ins/plugins/analysis/core/filter/RegexpFilter.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...lugins/analysis/core/model/ReportScanningTool.java	84.05% <ø> (ø)	19.00 <0.00> (ø)
...a/io/jenkins/plugins/analysis/core/model/Tool.java	74.28% <ø> (ø)	10.00 <0.00> (ø)
...ns/analysis/core/steps/AnalysisStepDescriptor.java	18.18% <ø> (+1.51%)	1.00 <0.00> (ø)
...ns/plugins/analysis/core/steps/IssuesRecorder.java	73.30% <ø> (+0.65%)	79.00 <0.00> (ø)
...lugins/analysis/warnings/axivion/AxivionSuite.java	3.48% <ø> (ø)	0.00 <0.00> (ø)
...lysis/warnings/axivion/RemoteAxivionDashboard.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...plugins/analysis/warnings/groovy/GroovyParser.java	73.72% <ø> (ø)	16.00 <0.00> (ø)
...ins/plugins/analysis/warnings/tasks/OpenTasks.java	40.29% <ø> (ø)	10.00 <0.00> (ø)

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