fix: warn on deprecated envFile

[SSDWGG]

What changed

• Fixes InlineConfig.envFile is deprecated but no runtime warning is emitted #22547.
• Emit a runtime deprecation warning when envFile: false is used.
• Keep the existing backward-compatible behavior that maps envFile: false to envDir: false.
• Add config tests covering the warning path and the non-warning envDir: false path.
• Note the deprecation in the migration guide.

Why

envFile is already marked deprecated in the type definition, but the runtime path stayed silent. This makes migration harder for programmatic API users and leaves behavior inconsistent with other deprecated Vite options that warn when used.

Impact

Users who still pass envFile: false now get a clear migration hint without changing behavior. Consumers already on envDir: false keep the same experience with no extra warning.

Validation

• pnpm vitest run packages/vite/src/node/__tests__/config.spec.ts -t "envFile option|envDir to disable env files"
• pnpm exec eslint packages/vite/src/node/config.ts packages/vite/src/node/__tests__/config.spec.ts
• pnpm exec oxfmt --check packages/vite/src/node/config.ts packages/vite/src/node/__tests__/config.spec.ts docs/guide/migration.md

[SSDWGG]

I checked the remaining red check. The Vite CI matrix is green; the failing Run zizmor job appears unrelated to this change and fails before auditing this PR because it cannot list tags for actions-cool/issues-helper in .github/workflows/issue-close-require.yml (403 Forbidden from GitHub/git-upload-pack).

[truffle-dev]

One follow-up from the empirical pass on the linked issue: at HEAD, @react-router/dev still passes envFile: false in two internal child-compiler invocations (plugin.ts:1552 and vite-node.ts:61), and both sites are intentional ("child compiler suppresses env loading because the parent already handled it"), not migration debt. So as-is this warning will fire on every React Router dev/build against vite 8.x until they swap to envDir: false.

Existing precedent for that situation is at config.ts:1207-1213 — the optimizeDeps.esbuildOptions warning uses a "You or a plugin you are using have set ..." framing so end users know the warning is plugin-side and to ignore it until their plugin migrates. Same framing here would keep parity:

if (config.envFile === false) {
  logger.warn(
    colors.yellow(
      `You or a plugin you are using have set \`envFile: false\` ` +
        `but this option is now deprecated. ` +
        `Please use \`envDir: false\` instead.`,
    ),
  )
}

Conditional on the sequencing question on #22547 going this direction (rather than coordinating a React Router PR first). Either way the framing tweak is small.