Consolidate repetitive warning messages

[apparentlymart]

Some of our warning messages are produced in response to the use of deprecated language constructs that might be used many times across a whole configuration, and so the resulting warnings can easily overwhelm the other parts of Terraform's output from commands like terraform plan.

As a compromise, we'll detect when the diagnostics includes more than two warnings with the same summary text and consolidate the third and any further warnings into a single count of additional warnings at the end of the second warning message, like this:

Warning: Interpolation-only expressions are deprecated

  on cli-consolidate-warnings.tf line 7, in resource "null_resource" "b":
   7:   triggers = "${map("a", "b")}"

Terraform 0.11 and earlier required all non-constant expressions to be
provided via interpolation syntax, but this pattern is now deprecated. To
silence this warning, remove the "${ sequence from the start and the }"
sequence from the end of this expression, leaving just the inner expression.

Template interpolation syntax is still used to construct strings from
expressions when the template includes multiple interpolation sequences or a
mixture of literal strings and interpolations. This deprecation applies only
to templates that consist entirely of a single interpolation sequence.

(and 4 more similar warnings elsewhere)

This is intended to strike a compromise between making sure these warnings remain visible but avoiding them distracting too much from other output that appears alongside them. This fixes #23406.

[ghost]

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