Section 90D1/2. Any retirement system of a city, town, county, region, district or authority may, upon the majority vote of the board of such system and by the local legislative body, increase the retirement allowance of any member of the retirement system, who has been retired under this chapter or similar provision of earlier law on a superannuation, accidental disability or ordinary disability retirement allowance and who has completed at least 25 years of creditable service, to an amount not to exceed $15,000. For the purposes of this section, a vote of the legislative body shall take place in the following manner: in a city, by a vote the city council subject to its charter; in a town, by a vote at a town meeting; in a county, by a vote of the county retirement board advisory council; in a region, by a vote of the regional retirement board advisory council; in a district, by a vote of the district members; and for an authority, by a vote of its governing body.