Address S4040: Strings should be normalized to uppercase

[bart-vmware]

Address existing violations of S4040: Strings should be normalized to uppercase 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:

• Change ToLower calls to ToUpper, consider ToUpperInvariant
• ~Suppress the violation in code using #pragma warning disable/restore, preceded by a justification comment if not obvious~

Related to #923.