Relating to the authority of the governing body of a taxing unit other than a school district to adopt an exemption from ad valorem taxation of a portion, expressed as a dollar amount, of the appraised value of an individual's residence homestead and to the authority of the governing body of any taxing unit that has adopted an exemption from ad valorem taxation of a percentage of the appraised value of an individual's residence homestead to reduce the amount of or repeal the exemption.