ci: update base image for GHA

[codecov[bot]]

Codecov Report

Merging #365 (7ef9112) into main (fead12b) will decrease coverage by 0.59%. The diff coverage is 100.00%.

:exclamation: Current head 7ef9112 differs from pull request most recent head 51b0968. Consider uploading reports for the commit 51b0968 to get more accurate results

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
- Coverage   89.35%   88.77%   -0.59%     
==========================================
  Files          98       97       -1     
  Lines        2761     2992     +231     
==========================================
+ Hits         2467     2656     +189     
- Misses        294      336      +42     

Impacted Files	Coverage Δ
...s/site/testing_storage/storage_integration_test.cc	78.57% <ø> (-1.21%)	:arrow_down:
.../integration_tests/cloud_event_integration_test.cc	89.15% <ø> (-2.79%)	:arrow_down:
google/cloud/functions/internal/base64_decode.cc	100.00% <ø> (ø)
...oud/functions/internal/parse_cloud_event_legacy.cc	95.94% <ø> (-1.79%)	:arrow_down:
google/cloud/functions/http_request.h	100.00% <100.00%> (ø)

... and 54 files with indirect coverage changes

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