Additional async-chores for SMTP/IMAP health checks

[peterwurzinger]

What this PR does / why we need it:
Some chores regarding async methods available in .NET Standard 2.0 and > .NET 5 and passing the cancellation token in operations that could potentially block as discussed in #1739 (comment)

Which issue(s) this PR fixes:
None, but related to #1738

Special notes for your reviewers:
This PR is based on branch https://github.com/peterwurzinger/AspNetCore.Diagnostics.HealthChecks/tree/fix/smtp-timeout
It doesn't make sense to base it on main, since it would result in merge conflicts, assuming that the fix in #1738 would get merged first anyways.

Does this PR introduce a user-facing change?:
No, it only introduces binary breaking changes which shouldn't be a problem according to #1739 (comment)

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Unit tests passing
• End-to-end tests passing

[sungam3r]

OK, resolve conflicts and I will review.

[peterwurzinger]

@sungam3r Resolved

[sungam3r]

Thanks.

[sungam3r]

This PR was merged without running tests, see #1018.

[peterwurzinger]

This PR was merged without running tests, see #1018.

Is there something broken or to do? I ran manual tests and unit tests against a SMTP/IMAP implementation (https://github.com/rnwood/smtp4dev), which doesn't support (STAR)TLS, but basic SMTP and IMAP functionalityand coudln't observe any errors that would not be expected.

[sungam3r]

Test step in network CI worflow is disabled https://github.com/Xabaril/AspNetCore.Diagnostics.HealthChecks/blob/master/.github/workflows/healthchecks_network_ci.yml#L83-L97

I think this is because not properly configured services https://github.com/Xabaril/AspNetCore.Diagnostics.HealthChecks/blob/master/.github/workflows/healthchecks_network_ci.yml#L32-L62