Enforce a character limit of 2 in autocomplete search

What does this MR do and why?

From Add min char limitation to search autocomplete ... (#403325 - closed):

Enforce the min character limit for resources autocomplete data since that hits the database and Elasticsearch. The search_autocomplete_opts method has three filter types (search, generic, and default) which return a variety of data.

• Combine the two character limits used in search params and search abuse detection
  • remove Gitlab::Search::AbuseDetection::MINIMUM_SEARCH_CHARS
  • replace use with Gitlab::Search::Params::MIN_TERM_LENGTH
  • change Gitlab::Search::Params::MIN_TERM_LENGTH to 2
• The resource_results method should return an empty array if the term length is less than the min (use Gitlab::Search::Params::MIN_TERM_LENGTH)

Screenshots or screen recordings

Before

Now

How to set up and validate locally

1. Type 1 character into the search bar and note that nothing is returned for projects, groups, issues and users.
2. Type 2 characters into the search bar and note that results are returned for projects, groups, issues and users.
3. Type 3 characters into the search bar and note that results are returned for projects, groups, issues and users.

MR acceptance checklist

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

Related to #403325 (closed)