refactor(schematics): replace deprecated TestBed.get

[LayZeeDK]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https:/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The deprecated TestBed.get method is used in generated test suites.

Closes #2498

What is the new behavior?

The strongly typed TestBed.inject method is used in generated test suites.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Cc @timdeschryver

[ngrxbot]

Preview docs changes for 0c15a79 at https://previews.ngrx.io/pr2503-0c15a79/

[brandonroberts]

Thanks @LayZeeDK!