fix: fix incorrect .Is() method on update skipped due to backoff error

[pmalek]

What this PR does / why we need it:

This fixes an issue where KIC crash due to a stack overflow when a broken config is applied in dbmode.

The problematic piece was:

func (e ErrUpdateSkippedDueToBackoffStrategy) Is(err error) bool {
	return errors.Is(err, ErrUpdateSkippedDueToBackoffStrategy{})
}

which didn't work as intended. .Is() is meant to compare errors, as in compare their values, not types.

Where this was called we intended to check the type of an error, not it's value.

The stack overflow is prevent by removing the .Is() method from UpdateSkippedDueToBackoffStrategyError as it's not needed now.

The test that failed due to a stack overflow before this change was introduced and passes now:

	skippedErr := sendconfig.NewUpdateSkippedDueToBackoffStrategyError("reason")

	t.Run("errors.Is()", func(t *testing.T) {
		assert.False(t,
			errors.Is(skippedErr, deckerrors.ConfigConflictError{
				Err: sendconfig.NewUpdateSkippedDueToBackoffStrategyError("different reason"),
			}),
			"shouldn't panic when using errors.Is() with NewUpdateSkippedDueToBackoffStrategyError",
		)
	})

Which issue this PR fixes:

Fixes: #4212

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Patch coverage: 50.0% and project coverage change: -0.1 ⚠️

Comparison is base (7f4ab19) 62.6% compared to head (76f1a3c) 62.5%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4213     +/-   ##
=======================================
- Coverage   62.6%   62.5%   -0.1%     
=======================================
  Files        153     153             
  Lines      17008   17006      -2     
=======================================
- Hits       10652   10644      -8     
- Misses      5686    5694      +8     
+ Partials     670     668      -2     

Impacted Files	Coverage Δ
internal/dataplane/kong_client.go	66.8% <0.0%> (ø)
internal/dataplane/sendconfig/sendconfig.go	78.3% <0.0%> (ø)
internal/dataplane/sendconfig/backoff_strategy.go	84.6% <75.0%> (+6.0%)	⬆️

... and 4 files with indirect coverage changes

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