Upgrading go version to 1.21

[darshan-sj]

Upgraded go version to 1.21

Tested:

1. go get ./...
2. go mod tidy
3. built and ran a sharded migration from mysql to spanner and cleaned up the resources.
4. built and ran a non sharded migration from mysql to spanner and cleaned up the resources.
5. No errors in the SMT log in all the above steps.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.17%. Comparing base (b7ad348) to head (7bf6c80).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #959   +/-   ##
=======================================
  Coverage   46.17%   46.17%           
=======================================
  Files         169      169           
  Lines       22036    22036           
  Branches      515      515           
=======================================
  Hits        10176    10176           
  Misses      11275    11275           
  Partials      585      585           

Components	Coverage Δ
backend-apis	45.37% <ø> (ø)
backend-library	52.28% <ø> (ø)
cli	5.05% <ø> (ø)
frontend	33.95% <ø> (ø)

[shreyakhajanchi]

We should verify that the version changes are compatible with the kokoro script in SMT GoB repository, ideally it would require to update the version in GoB release script first.
cc: @manitgupta