[Validator] Rename issn constraint message option to be consistent with all constraints having a single message option
merged branch ajgarlag/feature/issn-validator-message-option (PR #7872)

This PR was merged into the master branch.

Discussion
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[Validator] Rename issn constraint message option.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Rename issn constraint message option to be consistent with all constraints having a single message option as suggested by @stof at https://github.com/symfony/symfony/commit/a4abfb95509437153bee13f1a49ae74a63408806#L0R23

Commits
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57c724a [Validator] Rename issn constraint message option to be consistent with all constraints having a single message option