[release/2.0] Update .gitmodules: use tags spack-stack-2.0.0 for submodules

[climbfuji]

Description

For release/2.0: Update .gitmodules to use tags spack-stack-2.0.0 for submodules spack and spack-packages. No code changes.

Dependencies

None

Issues addressed

Working toward #1835

Applications affected

None

Systems affected

None

Testing

• CI: Note whether the automatic tests (GitHub actions tests that run automatically for every commit) pass or not
  • GitHub actions CI tests pass
  • GitHub actions CI tests do not pass (provide explanation)
  • GitHub actions CI tests skipped (provide explanation if necessary)
• New tests added: List and describe any new tests added to GitHub actions
  • ...
• Additional testing: Add information on any additional tests conducted
  • ...

Checklist

• This PR addresses one issue/problem/enhancement or has a very good reason for not doing so.~~
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.
• All necessary updates to the spack-stack wiki will be made when this PR is merged