Auto-update spec version

[chalin]

• Contributes to Add GH action to pull in latest release of the OTel specification #2387
• Adds config so that the spec version is checked in the same cron job as the other versions, and a PR is generated as necessary.
• Updates the spec submodule to match the latest version when there is an update.
• There are no changes to the generated site files despite the update to the spec submodule.

/cc @open-telemetry/specs-approvers

[chalin]

/cc @trask

[chalin]

Manually successfully triggered the updated action, https://github.com/open-telemetry/opentelemetry.io/actions/runs/4917014280/jobs/8781532074:

...
REPO:            opentelemetry-specification
LATEST VERSION:  1.20.0
SEARCHING for:   '^ *spec:' in scripts/content-modules/adjust-pages.pl
CURRENT VERSION:   spec: 1.20.0
+ sed -i.bak -e 's/\(^ *spec:\) .*/\1 1.20.0/' scripts/content-modules/adjust-pages.pl
Already at the latest version. Exiting