CI: migrate to golangci-lint v2

[cce]

Summary

Following the upgrade to Go 1.25 in #6468 we should also migrate to golangci-lint v2, which supports the new Go version in its binaries.

Test Plan

Existing tests should pass, updated reviewdog job to use the new version.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.17%. Comparing base (e68b54e) to head (fcfe79a).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cmd/nodecfg/untar.go	0.00%	22 Missing ⚠️
shared/pingpong/pingpong.go	0.00%	2 Missing ⚠️
cmd/goal/node.go	0.00%	1 Missing ⚠️
cmd/nodecfg/download.go	0.00%	1 Missing ⚠️
cmd/opdoc/opdoc.go	0.00%	1 Missing ⚠️
network/gossipNode.go	0.00%	1 Missing ⚠️
network/wsPeer.go	0.00%	1 Missing ⚠️
util/metrics/reporter.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6473      +/-   ##
==========================================
- Coverage   46.37%   46.17%   -0.21%     
==========================================
  Files         668      660       -8     
  Lines      112102   111965     -137     
==========================================
- Hits        51989    51697     -292     
- Misses      57368    57511     +143     
- Partials     2745     2757      +12     

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[Copilot]

Pull Request Overview

This PR migrates the project from golangci-lint v1 to v2 to support Go 1.25. The migration involves updating the linter configuration format, converting a custom plugin from v1's shared object to v2's module plugin interface, and updating CI workflows.

Key Changes:

• Upgraded golangci-lint from v1.64.8 to v2.5.0
• Migrated .golangci.yml and .golangci-warnings.yml to v2 configuration format
• Converted partitiontest_linter from plugin to v2 module plugin architecture

Reviewed Changes

Copilot reviewed 39 out of 41 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scripts/buildtools/versions	Updated golangci-lint version and import path
scripts/buildtools/install_buildtools.sh	Updated golangci-lint module path
.golangci.yml	Migrated linter configuration to v2 format
.golangci-warnings.yml	Migrated warnings configuration to v2 format
.custom-gcl.yml	Added v2 custom plugin configuration
.github/workflows/reviewdog.yml	Updated CI workflow for v2 and simplified linter build process
cmd/partitiontest_linter/linter.go	Added v2 module plugin registration
cmd/partitiontest_linter/plugin/plugin.go	Removed obsolete v1 plugin code
cmd/partitiontest_linter/go.mod	Updated dependencies for v2 plugin system
Makefile	Updated linter build commands for v2
Multiple Go files	Applied linter auto-fixes (gosec nolints, code simplifications)

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