ENV VAR `FRONTEND_SEARCH_MIN_LENGTH`has a misleading name

The environment variable `FRONTEND_SEARCH_MIN_LENGTH` currently has a name that could lead to confusion. It suggests that it defines the minimum length of the search term for file searches (our prominent search in the top bar). However, its actual functionality is to set the minimum input length for identifying share receivers. To prevent any ambiguity, I propose renaming this variable to more accurately reflect its purpose.

For reference, the current usage of this variable is documented here:
[Frontend Service List - ownCloud Documentation](https://stackoverflow.com/questions/38886678/minlength-validation-in-form-is-not-working)