fix(billing): Use sentence case for on-demand budget terms

Addresses PR feedback to keep budget term capitalization in sentence case
for step titles and cart text, except when at the beginning of a sentence.

Author: dashed

static/gsApp/views/amCheckout/cart.tsx

@@ -239,7 +239,7 @@ function ItemsSummary({activePlan, formData}: ItemsSummaryProps) {
                               budgetTerm:
                                 activePlan.budgetTerm === 'pay-as-you-go'
                                   ? 'PAYG'
-                                  : displayBudgetName(activePlan, {title: true}),
+                                  : displayBudgetName(activePlan),
                             })
                           ) : (
                             // "Unlock with on-demand" gets cut off in non-chonk theme
@@ -248,7 +248,7 @@ function ItemsSummary({activePlan, formData}: ItemsSummaryProps) {
                                 budgetTerm:
                                   activePlan.budgetTerm === 'pay-as-you-go'
                                     ? 'PAYG'
-                                    : displayBudgetName(activePlan, {title: true}),
+                                    : displayBudgetName(activePlan),
                               })}
                             </Text>
                           )}

static/gsApp/views/amCheckout/index.spec.tsx

@@ -29,8 +29,8 @@ function assertCheckoutV3Steps(tier: PlanTier) {
   [
     'Build your plan',
     [PlanTier.AM1, PlanTier.AM2].includes(tier)
-      ? /Set your On-Demand limit/
-      : /Set your Pay-as-you-go limit/,
+      ? /Set your on-demand limit/
+      : /Set your pay-as-you-go limit/,
     'Pay monthly or yearly, your choice',
     'Edit billing information',
   ].forEach(step => {

static/gsApp/views/amCheckout/steps/setSpendLimit.tsx

@@ -56,7 +56,7 @@ function SetSpendCap({
         organization={organization}
         header={
           <StepHeader
-            title={t('Set your %s limit', displayBudgetName(activePlan, {title: true}))}
+            title={t('Set your %s limit', displayBudgetName(activePlan))}
             isActive
             stepNumber={stepNumber}
             isCompleted={false}