refactor: drop CLUSTER_ERA env var

[mkoura]

CLUSTER_ERA was always "conway"; replace with hardcoded value. CI input renamed to CI_CLUSTER_ERA, used only to gate PROTOCOL_VERSION=11 in runner/regression.sh.

[Copilot]

Pull request overview

This PR removes the CLUSTER_ERA environment variable from the test/CI plumbing by hardcoding the cluster era to conway, while keeping a CI-only CI_CLUSTER_ERA input to gate setting PROTOCOL_VERSION=11 in the regression runner.

Changes:

• Remove CLUSTER_ERA configuration/validation and references across scripts, docs, and pytest metadata.
• Hardcode cluster era to conway in runner scripts and default TESTNET_VARIANT to conway_fast when not bootstrapping.
• Rename CI input usage from CLUSTER_ERA to CI_CLUSTER_ERA for PV11 gating.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
scripts/setup_test_env.sh	Stops exporting CLUSTER_ERA in the generated activation script.
scripts/check_dev_env.sh	Removes printing of CLUSTER_ERA in dev env checks.
runner/regression.sh	Hardcodes cluster era and gates PROTOCOL_VERSION=11 via CI_CLUSTER_ERA.
runner/node_upgrade_pytest.sh	Hardcodes cluster era to conway for upgrade pytest runs.
runner/env_nightly_pv11	Drops CLUSTER_ERA from nightly env file.
runner/env_nightly_dbsync	Drops CLUSTER_ERA from nightly env file.
runner/env_nightly_cli	Drops CLUSTER_ERA from nightly env file.
runner/env_nightly	Drops CLUSTER_ERA from nightly env file.
README.md	Removes CLUSTER_ERA from documented env vars list.
cardano_node_tests/utils/configuration.py	Removes CLUSTER_ERA env handling and hardcodes default TESTNET_VARIANT.
cardano_node_tests/tests/conftest.py	Removes CLUSTER_ERA from pytest metadata stash.
.github/workflows/regression_reusable.yaml	Writes CI_CLUSTER_ERA instead of CLUSTER_ERA into CI environment.

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