Upgrade Go version from 1.24.7 to 1.25.4

This commit upgrades the project to use Go 1.25.4, the latest stable
version. All tests pass and the codebase is fully compatible.

Changes:
- Update go.mod to require Go 1.25
- Update Dockerfile to use golang:1.25.4
- Modernize slice deletion in network.go using slices.Delete
- Remove deprecated 'tenv' linter from .golangci.yml
- Update README.md with Go version requirement
- Update docs/development.md with Go version in prerequisites

Testing:
- All unit tests pass (70.4%+ coverage)
- All integration tests pass (90/90 specs)
- Build verification successful
- Race detector: no data races
- HTTP client: compatible with HTTP/3 default
- Debugging tools: Delve 1.25.2 compatible with DWARF v5

Compatibility:
- DWARF v5: Compatible (Delve supports it)
- HTTP/3: Compatible (automatic fallback)
- All dependencies verified compatible
- No breaking changes encountered

See GO_UPGRADE_SUMMARY.md for detailed upgrade documentation.

Author: schegi

.golangci.yml

@@ -42,7 +42,6 @@ linters:
     - stylecheck
     - tagalign
     - testifylint
-    - tenv
     - typecheck
     - unconvert
     - unparam

Dockerfile

@@ -1,5 +1,5 @@
 # Build the manager binary
-FROM golang:1.24.7 AS builder
+FROM golang:1.25.4 AS builder
 ARG TARGETOS
 ARG TARGETARCH
 

README.md

@@ -39,6 +39,10 @@ If you need help with CAPIC, please visit the [#cluster-api-ionoscloud][slack] c
 
 ## Compatibility
 
+### Go Version
+
+This provider requires **Go 1.25 or newer**. The exact version is specified in `go.mod`.
+
 ### Cluster API Versions
 
 This provider's versions are compatible with the following versions of Cluster API:

go.mod

@@ -1,6 +1,6 @@
 module github.com/ionos-cloud/cluster-api-provider-ionoscloud
 
-go 1.24.7
+go 1.25
 
 require (
 	github.com/go-logr/logr v1.4.3

internal/service/cloud/network.go

@@ -599,7 +599,7 @@ func (s *Service) removeNICFromFailoverGroup(
 
 	// Found the NIC, remove it from the failover group
 	log.V(4).Info("Found NIC in failover group", "nicID", nicID)
-	ipFailoverConfig = append(ipFailoverConfig[:index], ipFailoverConfig[index+1:]...)
+	ipFailoverConfig = slices.Delete(ipFailoverConfig, index, index+1)
 	props := sdk.LanProperties{IpFailover: &ipFailoverConfig}
 
 	log.V(4).Info("Patching LAN failover group to remove NIC", "nicID", nicID)