fix: Switch template to default to net7.0

[nickrandolph]

GitHub Issue (If applicable): closes #Version 17.4.0

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

unoapp template defaults to net6.0

What is the new behavior?

unoapp template defaults to net7.0, so should be able to be built and run using standard install of VS 17.4.0

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

Other information

Internal Issue (If applicable):

[gitpod-io]

[jeromelaban]

We cannot do that yet, we need dotnet/android#7473 to be fixed.