Upgrade grunt devDependency from ^1.5.3 to ^1.6.1 across all packages

[Copilot]

grunt ^1.5.3 has known issues; this upgrades all packages to ^1.6.1.

Changes

• shared/package.json, ts-plugin/package.json, eslint-ts-plugin/package.json, package.json (root): bump grunt devDependency ^1.5.3 → ^1.6.1
• common/config/rush/npm-shrinkwrap.json: regenerated via rush update --full; grunt now resolves to 1.6.1

Original prompt

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This section details on the original issue you should resolve

<issue_title>[BUG] The plugins currently have a dependency on grunt ^1.5.3 which has known issues</issue_title>
<issue_description>Review the dependency on grunt 1.5.3 and upgrade to 1.6.1, consider upgrading to at least grunt 1.6.1 and the related dependencies for all packages.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [BUG] The plugins currently have a dependency on grunt ^1.5.3 which has known issues #411

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