chore: downgrade typescript version

[alex-sl-eng]

JIRA issue URL: paste URL here

downgrade typescript version due to memory issue:

PR when upgrading: #1022
Related discussion: microsoft/TypeScript#17112 (comment)

Checklist

• JIRA link
• Check target branch
• Make sure all commit statuses are green or otherwise documented reasons to ignore
• QA needs to evaluate against the JIRA ticket
• Changed files and commits make sense for this PR

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[zubot]

This change is 

[djansen-redhat]

@aeng @seanf Shouldn't we have some Jira issue, details or otherwise to record why this was necessary?

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@69ebf6b). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1031   +/-   ##
=========================================
  Coverage          ?   36.99%           
  Complexity        ?     6153           
=========================================
  Files             ?     1647           
  Lines             ?    65376           
  Branches          ?     7615           
=========================================
  Hits              ?    24184           
  Misses            ?    39094           
  Partials          ?     2098

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[seanf]

@aeng @djansen-redhat See #1032 for another experiment

[seanf]

@aeng The first master build I can find which failed due to "JavaScript heap out of memory" was for this commit: 8b50de8

Four days before TypeScript 3 went in. So TS 3 is not the culprit at all.

[alex-sl-eng]

See #1032 for solution.