[8.19] (backport #8326) fix(ci): restore go mod tidy in otel-update.sh script

[mergify]

Without it, mage notice fails, because mage tidy fails, because magefile cannot be compiled, because the root go.mod file that Mage depends on is out of date.

Related to #8279, discussed with @pkoutsovasilis.

Steps to reproduce the issue

1. Pull latest main
2. Run .ci/scripts/otel-update.sh v0.127.0 v1.33.0, or make some other change in the root go.mod that invalidates go.sum.

Expected result:

• The otel-update.sh scripts / mage tidy succeeds

Actual result:

• The otel-update.sh script fails with error compiling magefiles

$ .ci/scripts/otel-update.sh v0.127.0 v1.33.0
=> Updating core from v0.123.0/v1.29.0 to v0.127.0/v1.33.0
=> Updating contrib from v0.123.0 to v0.127.0
=> Running mage notice
internal/pkg/config/config.go:13:2: missing go.sum entry for module providing package go.opentelemetry.io/collector/confmap (imported by github.com/elastic/elastic-agent/internal/pkg/config); to add:
        go get github.com/elastic/elastic-agent/internal/pkg/config
Error: error compiling magefiles
$
<hr>This is an automatic backport of pull request #8326 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @andrzej-stencel? 🙏