fix(nextcloud): make previews.max_allowed_resolution string type (#10748)

**Description**
I see an error when upgrading nextcloud instance: `[EINVAL]
values.nextcloud.previews.max_allowed_resolution: Not an integer`
⚒️ Fixes  # <!--(issue)-->

**⚙️ Type of change**

- [ ] ⚙️ Feature/App addition
- [x] 🪛 Bugfix
- [ ] ⚠️ Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] 🔃 Refactor of current code

**🧪 How Has This Been Tested?**
I have not personally tested this because testing would be more complex
than warrented for the fix IMO

**📃 Notes:**
The options I saw for this fix were:

1. Update default value to `18`
2. Update type to `string`

I selected option 2 because the `imaginary` repo specifies `18.0` as the
default.

**✔️ Checklist:**

- [x] ⚖️ My code follows the style guidelines of this project
- [x] 👀 I have performed a self-review of my own code
- [x] #️⃣ I have commented my code, particularly in hard-to-understand
areas
- [ ] 📄 I have made corresponding changes to the documentation
- [x] ⚠️ My changes generate no new warnings
- [ ] 🧪 I have added tests to this description that prove my fix is
effective or that my feature works
- [x] ⬆️ I increased versions for any altered app according to semantic
versioning

**➕ App addition**

If this PR is an app addition please make sure you have done the
following.

- [ ] 🪞 I have opened a PR on
[truecharts/containers](https://github.com/truecharts/containers) adding
the container to TrueCharts mirror repo.
- [ ] 🖼️ I have added an icon in the Chart's root directory called
`icon.png`

---

_Please don't blindly check all the boxes. Read them and only check
those that apply.
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about and
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charts/stable/nextcloud/Chart.yaml

@@ -29,7 +29,7 @@ sources:
   - https://github.com/nextcloud/docker
   - https://github.com/nextcloud/helm
 type: application
-version: 21.0.17
+version: 21.0.18
 annotations:
   truecharts.org/catagories: |
     - cloud

charts/stable/nextcloud/questions.yaml

@@ -179,10 +179,11 @@ questions:
                     - variable: max_allowed_resolution
                       label: Max Allowed Resolution
                       schema:
-                        type: int
+                        type: string
+                        valid_chars: '^[0-9]{3}(\.[0-9]{2})?$'
                         show_if: [["imaginary", "=", true]]
                         required: true
-                        default: 18.0
+                        default: "18.0"
                     - variable: max_file_size_image
                       label: Max File Size Image
                       schema: