docs: correct the "git never garbage-collected" phrasing; tag fenced block with text

Addresses review feedback on #5599:

1. The phrase "git never garbage-collected them" was technically
   inaccurate -- git does not GC tracked files. The files remain on
   this branch because they are still tracked and nobody has removed
   them. Reworded accordingly.

2. The fenced code block listing .github/workflows/ was untyped,
   triggering markdownlint MD040. Added the `text` language tag.

Both flagged by CodeRabbit.

README.md

@@ -13,7 +13,7 @@ If you're looking for ProxySQL itself, switch to the default branch:
 
 Only one directory matters on this branch:
 
-```
+```text
 .github/workflows/
 ├── ci-trigger.yml              ← Entry point; babysits CI-builds
 ├── ci-builds.yml               ← Builds ProxySQL in Docker, caches artefacts
@@ -38,8 +38,8 @@ are what trigger on push/PR; the files here are the actual job bodies.
 
 All other files on this branch (source code, docs, test directories, etc.)
 are **vestigial**. They're snapshots from an old point in history that
-happen to still exist on this branch because git never garbage-collected
-them. **Do not edit them here.** Any changes to source code, TAP tests,
+remain on this branch because they're still tracked here and nobody
+has removed them. **Do not edit them here.** Any changes to source code, TAP tests,
 infra scripts, or documentation belong on `v3.0`.
 
 ---