fix: replace deprecated elements with equivalent up-to-date items

[srieteja]

Closes https://github.com/atsign-foundation/at_widgets/issues/843

- What I did

• Replaced deprecated elements in at_onboarding_flutter's example app with up-to-date design elements

- How I did it

• BackgroundColor in MaterialApp() was previously deprecated and now removed. The latest version's equivalent of BackgroundColor is called 'surface' which is now used in the examples
• Also Changed:

1. color: Theme.of(context).backgroundColor -> color: Theme.of(context).canvasColor

2. style: Theme.of(context).textTheme.bodyText1 -> style: Theme.of(context).textTheme.bodyMedium,

- How to verify it

• Static analysis should pass. Also, requires manual testing to ensure design elements are not broken

- Description for the changelog

fix: replace deprecated elements with equivalent up-to-date items

[cpswan]

@srieteja I've merged #850 as we don't need to have the beta tests passing (yet)

This one is also good to go once you've confirmed that the manual tests are OK.

[srieteja]

@cpswan this one's build has been failing since last night. Currently trying to get a clean build.

[gkc]

@cpswan this one's build has been failing since last night. Currently trying to get a clean build.

@srieteja which build are you referring to? Only the checks on the beta channel are failing on this PR

[srieteja]

@gkc I'm trying to build the code into an apk and run it in an emulator. To see the new changes are working just fine. This build has been failing for me.

[srieteja]

@cpswan this one has passed manual testing too. Thanks @CurtlyCritchlow for the help.