Update go directive to 1.21 and tidy module dependencies

[qqxhb]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

User Case Description

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Bump Go directive to 1.21 and run full go mod tidy

The PR only touches module metadata. It replaces the placeholder go 1.24.0 directive with go 1.21 in go.mod and examples/go.mod, then re-tidies both modules. This drops many newer/unused checksums and re-adds versions that build under Go 1.21, aligning the dependency graph with the chosen toolchain.

Key Changes

• Changed go directive from 1.24.0 ➜ 1.21 in go.mod and examples/go.mod
• Replaced preview dependencies (golang.org/x/* v0.28+, v0.37.0, etc.) with older releases compatible with 1.21 (v0.17.0, v0.21.1-*, etc.)
• Downgraded github.com/spf13/pflag to v1.0.6; removed explicit indirects no longer needed; added new indirects (golang.org/x/exp, golang.org/x/text, etc.)
• Pruned unused checksums from go.sum / examples/go.sum; added checksums for the new/updated versions

Affected Areas

• go.mod
• go.sum
• examples/go.mod
• examples/go.sum

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This summary was automatically generated by @propel-code-bot