Address var-name linter warnings

[ivanvc]

What would you like to be added?

As discussed at the March 7th etcd's community meeting, enabling the var-name revive linter involves about 1,000 lines of changes. To break this down into smaller subtasks, this would be small enough to be reviewable in pull requests while keeping the codebase functional.

• client/ directory - client: address golangci var-naming issues #17582
• api/ directory - Two public exported variables that need to be changed (ErrGRPCClusterIdMismatch and ErrClusterIDMismatch). Does it make sense to deprecate the variables? Or, given that it's the main branch, is it ok to rename them? - api: address golangci var-naming issues #17585
• pkg/ directory - pkg: address golangci var-naming issues #17584
• server/ directory - This contains about 500 instances that need to be changed
  • server/etcdserver - etcdserver: address var naming lint rule #17614
  • server/storage - server/storage: address golangci var-naming issues #17660
  • server/config - server/config: address golangci var-naming issues #17854
  • server/embed
    • server/embed: address golangci var-naming issues #17674
    • server/embed: address golangci var-naming issues #17820
  • server/* - server: address golangci var-naming issues #17624
• tests/ directory - This contains about 350 instances that need to be changed
  • tests/common, tests/integration - tests/common, tests/integration: Address tests var-naming lint rule #17635
  • tests/e2e - tests/e2e: address golangci var-naming issues #17636
  • tests/framework - tests/framework: address golangci var-naming issues #17651
    • removal of (*Member) GRPCURL() tests/framework: address golangci var-naming issues [remove (*Member) GRPCURL()] #17655
  • tests/robustness - tests/robustness: address golangci var-naming issues #17654
• etcdutl/, tools/, tests/common/ - Address remaining var naming linter warnings #17863
• Fix deprecation comments, see: server/embed: address golangci var-naming issues #17674 (comment) - Fix deprecation comments from addressing var-naming work #17875
• Enable lint rule in tools/.golangci.yaml - Enable var-naming linter rule #17921

Why is this needed?

To improve the quality of the code.

[serathius]

Thanks @ivanvc for your hard work!