[DateTimePicker] Change showToolbar default value to false

[alisasanib]

• I have followed (at least) the PR section of the contributing guide.

Fixes mui/mui-x#4455

Problem:

The default value of the showToolbar prop was true while the according to the documentation suggestion, it should be false.

Solution:

• Changing the showToolbar value to false
• passing showToolbar as a prop to the previous tests that were failed by changing showToolbar to false.
• adding new tests to DateTimePicker.test.tsx and MobileDateTimePicker.test.tsx in order to check if the toolbar is rendered properly when showToolbar value is passed.

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against e2be163

[siriwatknp]

👋 The migration PR has been merged.

Please follow these steps to make sure that the contents are all updated. (Sorry for the inconvenience)

1. pull latest master from upstream to your branch
2. if your PR has changes on the *.md or demo files, you are likely to encounter conflict because all of them have been moved to the new folder.
   2.1 revert the change on those markdown files you did
   2.2 pull latest master from upstream (you should not see conflict)
   2.3 make the changes again at docs/data/material/*
3. run yarn docs:api
   • you might see the changes in docs/pages/material/api/* if your PR touches some of the components
   • it is okay if there is no changes

If you are struggle with the steps above, feel free to tag @siriwatknp

[mnajdova]

@alisasanib I will close this PR without merging as the pickers are moving inside https://github.com/mui/mui-x. Feel free to reopen it, once they are established there. cc @flaviendelangle