downgrade minimum go version and cleanup actions

[thaJeztah]

• relates to https://github.com/moby/moby/pull/50249#discussion_r2166191898

go.mod: don't enforce go1.23.5 patch version

commit 08fb086bc2859b5a5b29509ed8fead3421d781f9 (https://github.com/fluent/fluent-logger-golang/pull/131) added a go.mod, but set the minimum required version to the patch version.

Generally, go versions in go.mod, like other dependencies, follow "MVS" (minimum version selection) conventions, and specify the lowest possible / supported minimum version. Keeping the version low avoids unnecessary code churn in projects using this module.

gha: test against oldest supported and current go versions

Also set GOTOOLCHAIN to "local" to prevent changes in go.mod to override the version used in tests.

gha: simplify selection of branches

Omit selecting branches; by default any branch is included.

gha: set default permissions to "read" only

gha: set default timeout

GitHub's default timeout is 6 hours, which is really long. Set a more reasonable timeout to prevent "runnaway" runs from running for hours before timing out.

[thaJeztah]

cc @cosmo0920 ptal

[cosmo0920]

Thank you!