fix: remove support for google-api-core<1.26.0

[busunkim96]

Drop support for google-api-core < 1.26.0. This code was originally added in #774.

This should unblock the coverage failures in the generated showcase unit tests for #888.

[codecov]

Codecov Report

Merging #893 (738ca21) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #893 +/- ##
=========================================
Coverage 100.00% 100.00% =========================================
Files 26 27 +1 Lines 1608 1697 +89 Branches 328 347 +19 =========================================
+ Hits 1608 1697 +89 

Impacted Files	Coverage Δ
gapic/samplegen_utils/types.py	100.00% <ø> (ø)
gapic/samplegen_utils/utils.py	100.00% <ø> (ø)
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/samplegen/samplegen.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/metadata.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
... and 3 more

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[tseaver]

Would be nice to be able to drop support for the older google-auth versions, too.

[busunkim96]

@tseaver Definitely! I'm planning on a cutting a release for google-api-core that requires a newer google-auth version.