Bump next-babel-loader cache key (#24335)

This bumps the babel cache key since we modified the `react-loadable` babel plugin and we don't want any cached versions used since the module names generated in previous cached version won't match the newly expected values. 

x-ref: #24281 

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.

## Documentation / Examples

- [ ] Make sure the linting passes

packages/next/build/webpack/loaders/next-babel-loader.js

@@ -2,9 +2,9 @@ import { join } from 'path'
 import * as Log from '../../output/log'
 import babelLoader from './babel-loader/src/index'
 
-// increment 'o' to invalidate cache
+// increment 'p' to invalidate cache
 // eslint-disable-next-line no-useless-concat
-const cacheKey = 'babel-cache-' + 'o' + '-'
+const cacheKey = 'babel-cache-' + 'p' + '-'
 const nextBabelPreset = require('../../babel/preset')
 
 const customBabelLoader = babelLoader((babel) => {