rename-branches.js

const fetch = require('node-fetch');
const request = async (url, data = {}) => {
  const response = await fetch(`https://gitlab.com/api/v4/${url}`, {
    headers: {
      'PRIVATE-TOKEN': process.env.GITLAB_TOKEN,
      ...data.headers,
    },
    ...data,
  });
  return response;
};

const getProjects = async (id) => {
  const response = await request(`groups/${id}`);
  const project = response.json();
  return project;
};

const createBranch = async (id, ref, name) => {
  await request(
    `projects/${id}/repository/branches?branch=${name}&ref=${ref}`,
    {
      method: 'POST',
    }
  );
};

const defaultBranch = async (id, name) => {
  await request(`projects/${id}`, {
    method: 'PUT',
    body: `default_branch=${name}`,
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded',
    },
  });
};

const deleteBranch = async (id, ref) => {
  await request(`projects/${id}/repository/branches/${ref}`, {
    method: 'DELETE',
  });
};

async function main() {
  try {
    const { projects } = await getProjects(23);
    for (const { id, default_branch } of projects) {
      if (default_branch === 'master') {
        await createBranch(id, default_branch, 'main');
        await defaultBranch(id, 'main');
        await deleteBranch(id, 'master');
      }
    }
  } catch (e) {
    console.error(e);
  }
}

main();

You'd need to install npm i node-fetch before you can run this.