Update string for FxA signin onboarding header (#14132)

app/src/main/java/org/mozilla/fenix/home/sessioncontrol/viewholders/onboarding/OnboardingManualSignInViewHolder.kt

@@ -38,7 +38,7 @@ class OnboardingManualSignInViewHolder(view: View) : RecyclerView.ViewHolder(vie
 
     fun bind() {
         val context = itemView.context
-        headerText.text = context.getString(R.string.onboarding_firefox_account_header)
+        headerText.text = context.getString(R.string.onboarding_account_sign_in_header)
     }
 
     companion object {

app/src/main/res/layout/onboarding_manual_signin.xml

@@ -34,7 +34,7 @@
         app:layout_constraintEnd_toEndOf="parent"
         app:layout_constraintStart_toStartOf="@id/avatar_icon"
         android:layout_marginStart="52dp"
-        tools:text="@string/onboarding_firefox_account_header"
+        tools:text="@string/onboarding_account_sign_in_header"
     />
 
     <org.mozilla.fenix.utils.LinkTextView

app/src/main/res/values/strings.xml

@@ -972,8 +972,8 @@
     <string name="onboarding_whats_new_description">Have questions about the redesigned %s? Want to know what’s changed?</string>
     <!-- text for underlined clickable link that is part of "what's new" onboarding card description that links to an FAQ -->
     <string name="onboarding_whats_new_description_linktext">Get answers here</string>
-    <!-- text for the firefox account onboarding card header -->
+    <!-- text for the Firefox account onboarding sign in card header -->
-    <string name="onboarding_firefox_account_header">Start syncing bookmarks, passwords, and more with your Firefox account.</string>
+    <string name="onboarding_account_sign_in_header">Start syncing bookmarks, passwords, and more with your Firefox account.</string>
     <!-- Text for the button to learn more about signing in to your Firefox account -->
     <string name="onboarding_manual_sign_in_learn_more">Learn more</string>
     <!-- text for the firefox account onboarding card header when we detect you're already signed in to