RFR: 8280863: Update build README to reflect that MSYS2 is supported [v2]

[Magnus Ihse Bursie]

On Fri, 28 Jan 2022 08:49:56 GMT, TheShermanTanker <duke at openjdk.java.net> wrote:

>> Ever since [JDK-8257679](https://bugs.openjdk.java.net/browse/JDK-8257679) was merged in 16, MSys 2 became capable of building a functional JDK on Windows. This PR updates some outdated documentation in building.html and building.md to reflect that change.
>> 
>> I appreciate any reviews, and would like to know how I might be able to improve the wording of both updated docs, if possible.
>
> TheShermanTanker has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:
> 
>  - Clean up documentation and MSys 2 -> MSYS2
>  - Update documentation to mention MSys 2 support

You don't need to give the `/issue` command. That is only used if you want to resolve multiple issues with one fix. The issue number is extracted from the bug title. Now the same issue is listed twice (!). Try removing it by `/issue remove 8280863`

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PR: https://git.openjdk.java.net/jdk/pull/7242