fix: update default aux container limits and instance types

[erikwilson]

Description

Reduces our default MaxAuxContainersPerAgent values to 16, so containers would have ~1gb each on these 16gb machines. Also change the default aws instance types to non-burstable m5 instance.

Test Plan

Launch a lot of aux containers. They should not crash.

Commentary (optional)

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

Ticket

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.19%. Comparing base (63adae5) to head (6a7e5e2).
Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8959   +/-   ##
=======================================
  Coverage   47.19%   47.19%           
=======================================
  Files        1155     1155           
  Lines      175116   175116           
  Branches     2237     2237           
=======================================
  Hits        82648    82648           
  Misses      92310    92310           
  Partials      158      158           

Flag	Coverage Δ
backend	42.37% <ø> (ø)
harness	63.71% <100.00%> (ø)
web	42.53% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
harness/determined/deploy/gcp/constants.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[stoksc]

lgtm