docs: remove a1-standard-4 agent (#669)

## 📝 Description
<!-- Describe your changes in detail -->
Since `a1-standard-4` agents were deprecated, they have been removed
from the documentation.

## ✅ Checklist
- [ ] I have tested this change
- [x] This change requires documentation update

Author: bogyo210

docs/docs/reference/machine-types.md

@@ -120,15 +120,12 @@ Semaphore Cloud provides the following Apple machine types:
 
 | Type | Virtual CPUs | Architecture | OS Supported | Memory | Disk |
 |--|--|--|--|--|--|
-| `a1-standard-4` | 4 | x86_64 (AMD) | [macOS Xcode15](./os-apple#mac-15) | 8 | 150 |
-| `a2-standard-4` | 4 | Apple Silicon | [macOS Xcode15](./os-apple#mac-15)<br/>[macOS Xcode16](./os-apple#mac-16) | 8 | 150 |
-
-A1 machines can only be paired with [macOS Xcode15](./os-apple#mac-15).
+| `a2-standard-4` | 4 | Apple Silicon | [macOS Xcode16](./os-apple#mac-16)<br/>[macOS Xcode26](./os-apple#mac-26) | 8 | 150 |
 
 A2 machines can be paired with:
 
-- [macOS Xcode15](./os-apple#mac-15)
 - [macOS Xcode16](./os-apple#mac-16)
+- [macOS Xcode26](./os-apple#mac-26)
 
 ## See also
 

docs/docs/reference/os-apple.md

@@ -354,7 +354,7 @@ Following gems are pre-installed:
 </div>
 </details>
 
-## macOS Xcode 15 {#mac-15}
+## (DEPRECATED) macOS Xcode 15 {#mac-15}
 
 <Tabs groupId="editor-yaml">
 <TabItem value="editor" label="Editor">

docs/docs/reference/pipeline-yaml.md

@@ -95,7 +95,7 @@ Part of the [`agent`](#agent) definition. This is an optional property to specif
 If a value is not provided, the default for the machine type is used:
 
 - `e1-standard-*` machine types: `ubuntu2004`
-- `a1-standard-*` machine types: `macos-xcode15`
+- `a2-standard-*` machine types: `macos-xcode16`
 
 The list of valid values for Semaphore Cloud is available on the [machine types reference](./machine-types) page.
 
@@ -758,8 +758,8 @@ blocks:
    # highlight-start
       agent:
           machine:
-            type: a1-standard-4
-            os_image: macos-xcode15
+            type: a2-standard-4
+            os_image: macos-xcode16
    # highlight-end
       jobs:
         - name: Using agent job

docs/docs/using-semaphore/jobs.md

@@ -991,8 +991,8 @@ blocks:
     task:
       agent:
         machine:
-          type: a1-standard-4
-          os_image: macos-xcode15
+          type: a2-standard-4
+          os_image: macos-xcode16
     # highlight-end
       jobs:
         - name: Build

docs/versioned_docs/version-CE/reference/pipeline-yaml.md

@@ -769,8 +769,8 @@ blocks:
    # highlight-start
       agent:
           machine:
-            type: a1-standard-4
-            os_image: macos-xcode15
+            type: a2-standard-4
+            os_image: macos-xcode16
    # highlight-end
       jobs:
         - name: Using agent job

docs/versioned_docs/version-CE/using-semaphore/jobs.md

@@ -922,8 +922,8 @@ blocks:
     task:
       agent:
         machine:
-          type: a1-standard-4
-          os_image: macos-xcode15
+          type: a2-standard-4
+          os_image: macos-xcode16
     # highlight-end
       jobs:
         - name: Build