Switch to the schema generator that was moved into elastic org

[aleksmaus]

What is the problem this PR solves?

Switches to generator that we customized for fleet server and that now has a new home @ github.com/elastic/go-json-schema-generate, previously it was at github.com/aleksmaus/generate

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[mergify]

This pull request does not have a backport label. Could you fix it @aleksmaus? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v/d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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Build stats

• Reason: null

• Start Time: 2022-01-27T15:00:47.257+0000

• Duration: 74 min 38 sec

• Commit: 029e287

Test stats 🧪

Test	Results
Failed	0
Passed	230
Skipped	0
Total	230

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[scunningham]

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