Address S4060: Non-abstract attributes should be sealed

[bart-vmware]

Address existing violations of S4060: Non-abstract attributes should be sealed in the codebase and set severity to Warning in Steeltoe.Debug.ruleset and Steeltoe.Release.ruleset.

To find existing violations, enable the rule (see above) and rebuild src/Steeltoe.All.sln to make them appear in the Output window.

To address the violations, choose from the following on a case-by-case basis:

• Fix the violation by changing the code to not violate the rule
• Suppress the violation in code using #pragma warning disable/restore, preceded by a justification comment if not obvious