[jdk11u] RFR: 8251551: Use .md filename extension for README [v2]

[Paul Hohensee]

On Mon, 20 Jun 2022 08:29:46 GMT, George Adams <duke at openjdk.org> wrote:

>> backports https://bugs.openjdk.org/browse/JDK-8251551 as it's a low risk change
>
> George Adams has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:
> 
>   Backport 6ed221cb9ad2e81d92dda0ef32095dda5d52cb85

Unless this is a critical fix, it needs to be against jdk11u-dev rather than jdk11u.

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PR: https://git.openjdk.org/jdk11u/pull/47