platform_releases: fix markdown lint in opam 2.5.2 announcement

Resolve markdownlint-cli2 failures matching the 2.5.1 announcement style:

- MD012: drop duplicate blank lines
- MD026: remove trailing "!" from the "Try it" heading
- MD040: tag fenced code blocks (shell / powershell)
- MD029: indent the code blocks under list item 1 so the ordered
  list stays continuous (1. then 2.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: cuihtlauac

data/platform_releases/opam/2026-07-09-opam-2-5-2.md

@@ -17,45 +17,39 @@ We are pleased to announce the release of opam 2.5.2 fixing a security issue ([O
 
 We advise everyone to upgrade. Please read on for installation and upgrade instructions.
 
-
 ## Security fix
 
-
 * [OSEC-2026-10](https://osv.dev/vulnerability/OSEC-2026-10) / CVE-2026-57825: Fix a bug that allowed a package to install files anywhere on the system using a symlink to an external directory without warning the user and asking for their permission ([#7005](https://github.com/ocaml/opam/pull/7005))
 
 Distributions maintainers that have not already done so, are invited to either upgrade their opam package to 2.5.2 or backport the fix. For any questions please send an email to the authors of this here blog post as listed above.
 
-
 ## Other changes
 
 * Re-allow `..` in `.install` files, partially reverting 2.5.1's [#6898](https://github.com/ocaml/opam/pull/6898) ([#7009](https://github.com/ocaml/opam/pull/7009), [ocaml/dune#14393](https://github.com/ocaml/dune/issues/14393))
 
-
-## Try it!
+## Try it
 
 The upgrade instructions are unchanged:
 
 1. Either from binaries: run
 
-For Unix systems
-```
-bash -c "sh <(curl -fsSL https://opam.ocaml.org/install.sh) --version 2.5.2"
-```
-or from PowerShell for Windows systems
-```
-Invoke-Expression "& { $(Invoke-RestMethod https://opam.ocaml.org/install.ps1) } -Version 2.5.2"
-```
-or download manually from [the Github "Releases" page](https://github.com/ocaml/opam/releases/tag/2.5.2) to your PATH.
+    For Unix systems
+    ```shell
+    bash -c "sh <(curl -fsSL https://opam.ocaml.org/install.sh) --version 2.5.2"
+    ```
+    or from PowerShell for Windows systems
+    ```powershell
+    Invoke-Expression "& { $(Invoke-RestMethod https://opam.ocaml.org/install.ps1) } -Version 2.5.2"
+    ```
+    or download manually from [the Github "Releases" page](https://github.com/ocaml/opam/releases/tag/2.5.2) to your PATH.
 
 2. Or from source, manually: see the instructions in the [README](https://github.com/ocaml/opam/tree/2.5.2#compiling-this-repo).
 
-
 You should then run:
-```
+```shell
 opam init --reinit -ni
 ```
 
-
 Please report any issues to [the bug-tracker](https://github.com/ocaml/opam/issues).
 
 Happy hacking!