feat(gatsby): enable webpack caching for all (#32018)

Author: wardpeet

packages/gatsby/src/utils/flags.ts

@@ -79,11 +79,7 @@ const activeFlags: Array<IFlag> = [
     telemetryId: `FastDev`,
     experimental: false,
     description: `Enable all experiments aimed at improving develop server start time`,
-    includedFlags: [
-      `DEV_SSR`,
-      `PRESERVE_FILE_DOWNLOAD_CACHE`,
-      `PRESERVE_WEBPACK_CACHE`,
-    ],
+    includedFlags: [`DEV_SSR`, `PRESERVE_FILE_DOWNLOAD_CACHE`],
     testFitness: (): fitnessEnum => true,
   },
   {
@@ -144,13 +140,7 @@ const activeFlags: Array<IFlag> = [
     experimental: false,
     description: `Use webpack's persistent caching and don't delete webpack's cache when changing gatsby-node.js & gatsby-config.js files.`,
     umbrellaIssue: `https://gatsby.dev/cache-clearing-feedback`,
-    testFitness: (): fitnessEnum => {
-      if (sampleSiteForExperiment(`PRESERVE_WEBPACK_CACHE`, 20)) {
-        return `OPT_IN`
-      } else {
-        return true
-      }
-    },
+    testFitness: (): fitnessEnum => `LOCKED_IN`,
   },
   {
     name: `PRESERVE_FILE_DOWNLOAD_CACHE`,