eslint-plugin-jsx-a11y pin from Volto shouldn't have exact version pins

[ichim-david]

Running yarn why eslint-plugin-jsx-a11y returns ├─ @plone/volto@npm:17.15.0 │ └─ eslint-plugin-jsx-a11y@npm:6.7.1 [51660] (via npm:6.7.1 [51660]) │ ├─ eslint-config-react-app@npm:7.0.1 │ └─ eslint-plugin-jsx-a11y@npm:6.8.0 (via npm:^6.5.1)

We get the following error which breaks eslint testing Error: Plugin "jsx-a11y" was conflicted between ".eslintrc.js » ./node_modules/@plone/volto/.eslintrc" and ".eslintrc.js » ./node_modules/@plone/volto/.eslintrc » eslint-config-react-app".