Revise SSC GKV for self employed

[mjbloemer]

Bug description

• For self employed who are in GKV, is there really a general cap at 75% of Bezugsgröße? This is how it is modeled in https://github.com/iza-institute-of-labor-economics/gettsim/blob/b46acc07ae130215f7b4838acfd83dc4c1d2fd9e/gettsim/soz_vers.py#L117-L142
• This cap at 75% of Bezugsgröße might still be relevant for individuals who are not mainly self employed (less than 20 hour of work in self employed), see e.g.: https://selbststaendigkeit.de/sites/default/files/merkblatt_-_die_krankenversicherung_fuer_nebenberuflich_selbststaendige.pdf
• Mindestbemessungsgrundlage for self employed is missing, see § 240 SGB V (4). E.g. Mindestbemessungsgrundlage: 1.061,67 (from example) can be calculated as bezgr_w/90 * 30 = 3185/90 * 30 = 1061,67.

[ChristianZimpelmann]

Hi, @JHermann99 addressed this issue in #405 where we among other:

• remove cap at 75% of Bezugsgröße
• add Mindestbemessungsgrundlage

@mjbloemer, would you like to have a look over the PR?