RUBY-3797 Migrate away from set-temp-creds.sh

[comandeo-mongo]

What

Two related Evergreen/CI cleanups under RUBY-3797:

1. Migrate away from the deprecated set-temp-creds.sh (the original
   scope of this ticket), in preparation for its removal upstream
   (DRIVERS-3433).
2. Remove the MongoDB 4.2 build variants (folded in here). 4.2 is on
   the EOL track (RUBY-3811 deprecated it; RUBY-3819 covers the
   min-version bump to 4.4), and its variants are currently broken at
   server bootstrap (see note below).

set-temp-creds migration

The legacy csfle/set-temp-creds.sh is deprecated upstream and emits a
warning pointing at setup-secrets.sh. Most of the migration already
happened: run-tests.sh moved to setup-secrets.sh under RUBY-3886
(#3048), and setup_secrets.py now produces the same CSFLE_AWS_TEMP_*
credentials plus Azure/GCP access tokens.

The only remaining consumer was run-tests-serverless.sh, which is dead
code: serverless testing was removed in RUBY-3652 (#2942) and no Evergreen
task references it.

• Delete .evergreen/run-tests-serverless.sh (orphaned; last
  set-temp-creds.sh consumer).
• Fix three stale comments in spec/support/spec_config.rb that still
  referenced set-temp-creds.sh; the CSFLE_AWS_TEMP_* env vars they
  document are now produced by setup-secrets.sh.

After this, there are no remaining references to set-temp-creds.sh.

Remove MongoDB 4.2 variants

• Remove the 4.2 value from the mongodb-version axis.
• Remove 4.2 from the mongo-4.x and stress older matrices (4.4 kept).
• Regenerated .evergreen/config.yml via rake eg:build; rake eg:validate
  passes.

Note: the 4.2 (and 4.4) variants currently fail at bootstrap-mongo-orchestration
because drivers-evergreen-tools starts MongoDB >= 4.2 with mongodb-runner,
which pins mongodb-runner but not its transitive bson dependency, so the
recent bson@7.3.0 release crashes server startup. That root cause must be
fixed in drivers-evergreen-tools; removing 4.2 only reduces exposure for the
EOL version.

Test plan

CI/shell + Evergreen-config change. No runtime Ruby code is touched.

• rake eg:build regenerates config.yml; rake eg:validate passes
• bundle exec rubocop spec/support/spec_config.rb — no offenses
• git grep set-temp-creds — no references remain
• grep 4.2 .evergreen/config.yml — no 4.2 variants remain

Jira

https://jira.mongodb.org/browse/RUBY-3797

[Copilot]

Pull request overview

This PR updates the Ruby driver’s Evergreen/CI configuration to remove reliance on the deprecated set-temp-creds.sh helper and to drop MongoDB 4.2 build coverage (EOL), aligning the repo’s CI setup with current upstream tooling and supported server versions.

Changes:

• Updated FLE/CSFLE test configuration comments to reference setup-secrets.sh instead of set-temp-creds.sh.
• Removed the orphaned .evergreen/run-tests-serverless.sh script (previously the last consumer of set-temp-creds.sh).
• Removed MongoDB 4.2 from Evergreen axes/matrices and regenerated .evergreen/config.yml accordingly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
spec/support/spec_config.rb	Updates documentation comments to reflect the current secrets provisioning script (setup-secrets.sh).
.evergreen/run-tests-serverless.sh	Deletes dead serverless test runner script (and last set-temp-creds.sh usage).
.evergreen/config/standard.yml.erb	Removes MongoDB 4.2 from buildvariant matrices (mongo-4.x, stress older).
.evergreen/config/axes.yml.erb	Removes the 4.2 value from the mongodb-version axis.
.evergreen/config.yml	Regenerated Evergreen config reflecting removal of the 4.2 axis/matrix entries.

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