fix(application): use void when invoking bootstrap()

[yousefdawood7]

PR Checklist

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (not applicable)
• Docs have been added / updated (not applicable)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no API changes)
• Build related changes
• CI related changes
• Other

What is the current behavior?

The generated main.ts template invokes bootstrap() without handling the returned promise:

await bootstrap();

[kamilmysliwiec]

v12 uses await #2302