Upgrade mockery to v3

[pchila]

Mockery is used for generating mocks within elastic-agent repository.

At the time of writing, elastic agent uses mockery v2 but this version will not receive feature updates after 6 months from v3 release, although there is a commitment to support new go versions and bugfixes.

The goal of this issue is to switch from mockery v2 to mockery v3.

Definition of done:

• .mockery.yaml is migrated to v3 following the migration guide
• mocks are generated in a single file mocks_test.go in the same package as the interface definition/usage abandoning the parallel testing/mocks package tree.
• modify the dev:regenerateMocks mage target accordingly

[kaanyalti]

It would be nice if the mage command enforces the mockery version.