Use prefix search instead of ngrams for sha fields

What does this MR do?

Currently, SHAs are indexed using ngrams from 5 to 40 characters. This means that each SHA is split into 35 separate terms taking up a lot of storage. SHAs are quite unique from 4-5 characters on, so a simple prefix search will be sufficiently fast and as effective as ngrams with term matching.

This MR replaces current ngrams analyzers with prefix search.

Original issue

Testing different options on a project from gitlabhq_export.tar.gz

Options	Size, MB	%
ngrams	899.1	100.00%
prefix search	788.67	87.71%	-12.29%

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
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• [-] Database guides
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
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Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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