fix: lift root postcss to 8.5.23 (Dependabot) (#23396)

## Summary

Clears the root postcss alert
[1851](https://github.com/twentyhq/twenty/security/dependabot/1851):
**GHSA-r28c-9q8g-f849** (high) - path traversal in previous source map
auto-loading (`sourceMappingURL`) leading to arbitrary `.map` file
disclosure, vulnerable `<= 8.5.17`.

The root lockfile carried **two** vulnerable copies, both behind exact
pins with no fixed upstream release:

| Copy | Pinner | Latest release still pins |
|---|---|---|
| 8.5.15 | `next` (8.4.31 exact) | 16.2.12 -> 8.4.31 |
| 8.5.14 | `@mintlify/common` (8.5.14 exact) | 1.0.1051 -> 8.5.14 |

So no parent upgrade reaches the fix. The existing `next/postcss`
resolution moves **8.5.15 -> 8.5.23** and a matching
**`@mintlify/common/postcss`** pin is added, placed alphabetically among
the other `@mintlify/*` entries. The caret consumers (`^8.4.38`,
`^8.4.47`, `^8.5.15`) dedupe onto the same version.

## Verification

- The two copies **collapse into a single `postcss@8.5.23` entry**;
nothing at or below 8.5.17 remains.
- `yarn install --immutable` passes.
- 8.5.23 published 2026-07-24, clears the 3-day npm age gate.
- `//resolutions` updated with the advisory, both pinners (and their
latest-version evidence) and the drop condition.

The app-lockfile side of this advisory shipped separately in #23340.

yarn.lock

@@ -39345,12 +39345,12 @@ __metadata:
   languageName: node
   linkType: hard
 
-"nanoid@npm:^3.3.11, nanoid@npm:^3.3.12":
-  version: 3.3.12
-  resolution: "nanoid@npm:3.3.12"
+"nanoid@npm:^3.3.16":
+  version: 3.3.16
+  resolution: "nanoid@npm:3.3.16"
   bin:
     nanoid: bin/nanoid.cjs
-  checksum: 10c0/ba142b7b39e11e80c16dd74b0365d407880c87c1cf7e1480956981ae940ee36060fa5b6f092cd1e315184dd19244c657bd017d03327bd3c62247d691c5e8edfb
+  checksum: 10c0/bbf2dcffe22d2b62d16de2711752070b539c0f644c7916f823ad6521986b2078cbe524f2d6240f58c46e9141ea0c7b87a029e100c0f7f175228cacaf30e41bba
   languageName: node
   linkType: hard
 
@@ -42425,25 +42425,14 @@ __metadata:
   languageName: node
   linkType: hard
 
-"postcss@npm:8.5.14":
-  version: 8.5.14
-  resolution: "postcss@npm:8.5.14"
+"postcss@npm:8.5.23, postcss@npm:^8.4.38, postcss@npm:^8.4.47, postcss@npm:^8.5.15":
+  version: 8.5.23
+  resolution: "postcss@npm:8.5.23"
   dependencies:
-    nanoid: "npm:^3.3.11"
+    nanoid: "npm:^3.3.16"
     picocolors: "npm:^1.1.1"
     source-map-js: "npm:^1.2.1"
-  checksum: 10c0/48138207cf5ef5581be1bfe2cb65ccfe0ac75e43888ba045afc8ed6043d7b56aeb3b9a9fe5b353ff554be943cd0cc15d826ccb991525159175971e5ee8ab0237
-  languageName: node
-  linkType: hard
-
-"postcss@npm:8.5.15, postcss@npm:^8.4.38, postcss@npm:^8.4.47, postcss@npm:^8.5.15":
-  version: 8.5.15
-  resolution: "postcss@npm:8.5.15"
-  dependencies:
-    nanoid: "npm:^3.3.12"
-    picocolors: "npm:^1.1.1"
-    source-map-js: "npm:^1.2.1"
-  checksum: 10c0/7f2e63ae22fbe43aace1bf652bd99da4e90737c64194d49e51ddc9cd0f9e51ff2861a7d734379b494deffa03a880a5c65eec70bc29ee9ebaa7136dde3eee8f31
+  checksum: 10c0/ed714e635b330bb42666ecdd0c0b4f30224ded15b0b0052d6bc2b92832f2cf17d1c1141359453f96f0a84f8fbf4c405a74df187f559163b8f31131b2535455aa
   languageName: node
   linkType: hard