refactor: util/template for handlebars (#5878)

lib/config/__snapshots__/validation.spec.ts.snap

lib/config/validation.spec.ts

lib/config/validation.ts

lib/manager/regex/index.spec.ts

lib/manager/regex/index.ts

lib/util/template/index.ts

lib/workers/common.ts

lib/workers/pr/body/banner.ts

lib/workers/pr/body/changelogs.ts

lib/workers/pr/body/footer.ts

lib/workers/pr/body/index.ts

lib/workers/pr/body/notes.ts

lib/workers/pr/body/updates-table.ts

lib/workers/repository/updates/branchify.ts

lib/workers/repository/updates/generate.ts