Fix generated gdsl code display names

[mbxsuite]

Calling https://localhost/pipeline-syntax/gdsl creates a document that escapes forward slashes.

The use of groovy.json.StringEscapeUtils.escapeJavaScript(java.lang.String), will break parsing of the resulting content within an IDE (i.e. IntelliJ Idea), if described plugin steps have a forward slash in their displayName content.

plugin samples:

• step envVarsForTool of plugin "pipeline-model-definition" (contains single quote) Link
• step sshGet, sshPut of plugin "ssh-steps" (contain forward slash) Link

According to Groovy API doc of escapeJavaScript :

The only difference between Java strings and JavaScript strings is that in JavaScript, a single quote must be escaped.

which is not 100% true to their code, as it also escapes forward slashes, which in turn breaks the parsing.

So to have the displayName properly escaped, it is sufficient to switch to escapeJava(String) and also escape single quotes afterwards.

(Using escapeJavaStyleString(<String>, true, false) whould also be sufficient, but that worker function is marked private in groovy.json.StringEscapeUtils)

JENKINS-70755

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