Fix dependabot not working

[fasmat]

Dependabot is currently not working for spacemeshos/poet. To verify run:

go install github.com/dependabot/cli/cmd/dependabot@latest
dependabot update go_modules spacemeshos/poet

It will fail with the error:

updater | Dependabot encountered '1' error(s) during execution, please check the logs for more details.
updater | +-------------------------------+
updater | |            Errors             |
updater | +-------------------------------+
updater | | dependency_file_not_parseable |
updater | +-------------------------------+

The proposed change fixes that by adding a patch version to go.mod and updating ci.yml to still use the newest 1.21.x when building the source code.

[codecov]

Codecov Report

Merging #394 (a97c339) into develop (fb07952) will decrease coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop    #394     +/-   ##
=========================================
- Coverage     74.8%   74.7%   -0.2%     
=========================================
  Files           23      23             
  Lines         1898    1898             
=========================================
- Hits          1421    1418      -3     
- Misses         359     361      +2     
- Partials       118     119      +1     

see 1 file with indirect coverage changes