Fixed issues with Nx 17

[mapolonio]

Problem

In Nx 17 runTasksInSerial is no longer exported from @nx/workspace and instead it must be imported from @nx/devkit.

There is also an issue with addDependenciesToPackageJson that arises when a project using Nx 17 omits the --includeAxe flag when calling the nx-playwright generator. Looks like the undefined value is no longer supported.

Solution

• All runTasksInSerial imports now come from @nx/devkit
• addDependenciesToPackageJson call refactored so the axe-playwright is omitted instead of set to undefined when --includeAxe flag is missing
• Bumped pnpm to >= 8.9 in the engines section because CI was picking Node v20 and pnpm v7.x.x which resulted in a failed prepare step
• Added --e2eTestRunner=playwright and --nextAppDir=false flags to the "Test plugin" step in the PR validation as the --interactive=false option is not enough to suppress those two prompts

Useful documentation

• Contributing guidelines