ENV VAR `FRONTEND_SEARCH_MIN_LENGTH`has a misleading name

[AlexAndBear]

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

[kobergj]

This will require deprecating this envvar for at least one version. Any suggestion into what it should be renamed?

[AlexAndBear]

e.g SHARE_RECEIVER_MIN_INPUT_LENGTH

[rhafer]

Is this somewhat related to #9000. So if we change the naming here, we should use some OCIS_ name and use that variable in the graph service as well. The frontend and graph service should use the same variable to avoid stuff like described in #9000