Add a keyword mapping for username

What does this MR do and why?

Adds a second mapping for usernames in elasticsearch's user index so that we have access to a keyword version of usernames.

Since we haven't added any users to the production index, we can get this MR in now.

Screenshots or screen recordings

Mapping for username

Before:

After:

Fields for username

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After:

Releasing this MR

Because the user index already exists on staging and production (staging has 735 users but production has 0 users indexed), we would need a reindex on both environments to apply the mapping change.

Run:

• In staging: Elastic::ReindexingTask.create!(targets: %w[User])
• In production: Elastic::ReindexingTask.create!(targets: %w[User])

These should be very quick.

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 #359820 (closed)