docs: Standardize Go version requirement to 1.25.5+ throughout documentation

- Update CONTRIBUTING.md to require Go 1.25.5 or later
- Update README.md prerequisites
- Update GETTING-STARTED.md requirements
- Update Dockerfile examples to use golang:1.25-alpine
- Update issue templates to reflect supported Go versions
- Ensure consistency across all documentation files

All CI workflows already use go-version-file: go.mod so they
automatically pick up the correct version from go.mod.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -22,7 +22,7 @@ A clear and concise description of what you expected to happen.
 
 **Environment (please complete the following information):**
 - OS: [e.g. macOS 14.0, Windows 11, Ubuntu 22.04]
-- Go version: [e.g. 1.21.5]
+- Go version: [e.g. 1.25.5]
 - Library version: [e.g. v1.0.0, commit hash if using main branch]
 - SoundTouch device model: [e.g. SoundTouch 10, SoundTouch 20]
 - Device firmware version: [if known]

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -24,10 +24,10 @@ body:
       label: Go Version
       description: What version of Go are you using?
       options:
+        - "1.25.5+"
+        - "1.25"
+        - "1.24"
         - "1.23"
-        - "1.22"
-        - "1.21"
-        - "1.20"
         - "Other (please specify in description)"
     validations:
       required: true

.github/ISSUE_TEMPLATE/device_compatibility.md

@@ -76,7 +76,7 @@ soundtouch-cli --host 192.168.1.100 play start
 
 **Environment**
 - **OS**: [e.g. macOS 14.0, Windows 11, Ubuntu 22.04]
-- **Go version**: [e.g. 1.21.5]
+- **Go version**: [e.g. 1.25.5]
 - **Library version**: [e.g. v1.0.0, commit hash]
 - **Network setup**: [home WiFi, corporate, etc.]