Backport release/v6.6: Update go-releaser heading with experimental notice

[seidroid]

Backport of #3879 to release/v6.6.

[cursor]

PR Summary

Low Risk
Documentation-only change to the GitHub release header template with no runtime or build behavior impact.

Overview
Updates the GoReleaser release.header on GitHub releases so the top heading is Experimental Pre-built Binaries instead of Sei {{ .Tag }}.

Checksum instructions, static linux/amd64 binary notes, and the Docker image pointer are unchanged; only the heading text and level (# → ##) differ.

^{Reviewed by Cursor Bugbot for commit 5ade49b. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Aug 7, 2026, 4:04 PM

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Aug 7, 2026, 4:04 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.12%. Comparing base (43d1152) to head (5ade49b).

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v6.6    #3880      +/-   ##
================================================
- Coverage         58.99%   58.12%   -0.88%     
================================================
  Files              2225     2151      -74     
  Lines            183577   175009    -8568     
================================================
- Hits             108307   101728    -6579     
+ Misses            65525    64237    -1288     
+ Partials           9745     9044     -701     

Flag	Coverage Δ
sei-db	70.41% <ø> (ø)
sei-db-state-db	?

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see 74 files with indirect coverage changes

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