Resolve BAD events due to unsupported gitlab_experiment context properties

What does this MR do and why?

Resolves a recently-introduced bug which is causing (nearly?) all FE events which contain the gitlab_experiment context to be NOT VALID due to extra information which the context schema does not expect or allow.

A bit o’ background

• Fixed one issue with experiment data being put into the frontend view code too soon in the render cycle, meaning that later run experiments would not be present in that JS object
• That fix sourced the experiment data from a slightly different (preferred) place which includes more information than the previous one did (mainly the excluded property)
• The new bits of data (mainly the excluded key) aren’t compatible with the gitlab_experiment context schema, resulting in NOT VALID events
• This quick-fix uses an allow-list to collect that context data
• There are follow-up efforts to update the <code data-sourcepos="11:47-11:63">gitlab_experiment</code> context schema itself and to update which schema version is used in the gitlab application (link needed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #345622 (closed)

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