Renamed cohorts for experiment reactivateUsersExperimentMay25. (#6149)

Task/Issue URL:
https://app.asana.com/1/137249556945/task/1210391900429638?focus=true

### Description
Updated cohort names in `ReactivateUsersToggles.kt` to use camelCase
format:
- Changed `duckplayer_prompt` to `duckplayerPrompt`
- Changed `browser_prompt` to `browserPrompt`

### Steps to test this PR

_Cohort naming_
- [ ] Verify that the cohort names are correctly updated in the codebase
- [ ] Ensure that any existing references to these cohorts are updated
or handle both naming formats

### NO UI changes

---
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used, see below:
  - https://app.asana.com/0/0/1210391900429638

Author: anikiki

dax-prompts/dax-prompts-impl/src/main/java/com/duckduckgo/daxprompts/impl/ReactivateUsersToggles.kt

@@ -42,8 +42,8 @@ interface ReactivateUsersToggles {
 
     enum class Cohorts(override val cohortName: String) : CohortName {
         CONTROL("control"), // no Dax prompts
-        VARIANT_DUCKPLAYER_PROMPT("duckplayer_prompt"), // show Duck Player Dax Prompt
-        VARIANT_BROWSER_PROMPT("browser_prompt"), // show Browser Comparison Dax Prompt
+        VARIANT_DUCKPLAYER_PROMPT("duckplayerPrompt"), // show Duck Player Dax Prompt
+        VARIANT_BROWSER_PROMPT("browserPrompt"), // show Browser Comparison Dax Prompt
     }
 
     companion object {