RFR: 8280863: Update build README to reflect that MSYS2 is supported [v3]

[TheShermanTanker]

> Ever since [JDK-8257679](https://bugs.openjdk.java.net/browse/JDK-8257679) was merged in 16, MSYS2 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 This PR updates some outdated documentation in building.html and building.md to reflect that change.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 incrementally with one additional commit since the last revision:

  Update htmlm using pandoc.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7242/files
  - new: https://git.openjdk.java.net/jdk/pull/7242/files/e0a4896e..85024eb1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7242&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7242&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7242.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7242/head:pull/7242

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