Make sure process_signatures gets migrated during V2 migrations

[nmuesch]

What does this PR do?

Make sure that process_signatures gets migrated to the manifest V2 format when running the ddev meta manifest migrate command

Motivation

Final piece to fully support process_signatures in manifest V2

Additional Notes

I confirmed this works locally by running ddev -x meta manifest migrate activemq 2.0.0
and confirming the manifest included this section at the bottom of the integration asset:

...
      "process_signatures": [
        "activemq"
      ]
...

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #12589 (593ed2f) into master (a48b506) will not change coverage.
The diff coverage is n/a.

Flag	Coverage Δ
arangodb	98.21% <ø> (ø)
disk	89.42% <ø> (ø)
fluentd	94.77% <ø> (ø)
ibm_i	81.58% <ø> (ø)
linux_proc_extras	96.22% <ø> (ø)
mapr	82.70% <ø> (ø)
silk	93.33% <ø> (ø)
system_core	92.85% <ø> (ø)

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