Move to checkout@v6 to avoid the deprecated Node 20.

The v4 version of this action uses the deprecated Node 20, which
triggers several warnings in every CI job.

Tagging release v1.3

Tagging release v1.2

Tagging release v1.1

Tagging release v1.0

Tagging release v0.9

Tagging release v0.8

Tagging release v0.7

Tag cling release v0.6.

See https://github.com/root-project/cling/releases/tag/v0.6 for the release page

Tag cling release v0.5