Pin gts version to v6

[ryohidaka]

Overview

Pin the gts version to v6.

Reason

This is because the following error occurs when executing eslint:

> npm run license && eslint --fix --no-error-on-unmatched-pattern src/ test/


> sample@0.0.0 license
> license-check-and-add add -f license-config.json

No default format specified. Using {"prepend":"/*","append":"*/"} as backup
! Inserted license into 0 file(s)
Command succeeded

Oops! Something went wrong! :(

ESLint: 8.57.1

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'configs' -> object with constructor 'Object'
    |     property 'flat/recommended-module' -> object with constructor 'Object'
    |     property 'plugins' -> object with constructor 'Object'
    --- property 'n' closes the circle
Referenced from: /Users/.../sample/.eslintrc.json
    at JSON.stringify (<anonymous>)
    at /Users/.../sample/node_modules/@eslint/eslintrc/dist/eslintrc.cjs:2156:45
    at Array.map (<anonymous>)
    at ConfigValidator.formatErrors (/Users/.../sample/node_modules/@eslint/eslintrc/dist/eslintrc.cjs:2147:23)
    at ConfigValidator.validateConfigSchema (/Users/.../sample/node_modules/@eslint/eslintrc/dist/eslintrc.cjs:2177:84)
    at ConfigArrayFactory._normalizeConfigData (/Users/.../sample/node_modules/@eslint/eslintrc/dist/eslintrc.cjs:3019:19)
    at ConfigArrayFactory._loadConfigData (/Users/.../sample/node_modules/@eslint/eslintrc/dist/eslintrc.cjs:2984:21)
    at ConfigArrayFactory._loadExtendedShareableConfig (/Users/..../sample/node_modules/@eslint/eslintrc/dist/eslintrc.cjs:3285:21)
    at ConfigArrayFactory._loadExtends (/Users/.../sample/node_modules/@eslint/eslintrc/dist/eslintrc.cjs:3156:25)
    at ConfigArrayFactory._normalizeObjectConfigDataBody (/Users/.../sample/node_modules/@eslint/eslintrc/dist/eslintrc.cjs:3095:25)

The error was caused by the difference in format, as gts v7 changed the format of eslint configuration files to a newer one (Flat config).

• Release Note for v7

Since gts v6 uses the previous format, it does not cause an error, so we will downgrade.

How to reproduce

1. Create a project using npx @google/aside init.

2. Run npm run lint.