new release v12.1.0

[jbathegit]

What needs to be done:

• increment number in VERSION file
• update release notes, in both GitHub and in ReleaseNotes.md
• move the existing html to previous_versions/v12.0.1, and add a new reference link for it within user_guide.md
• generate docs and update gh-pages branch with the new html

[jbathegit]

FYI, the VERSION file has already been incremented to 12.1.0 in the develop branch, re: #554

[jbathegit]

For this specific release, also update README.MD with the following:

• In the directive "Both path1 and path2 may be full or relative pathnames on the system, up to a maximum of 90 characters each.", change 90 to 240.
• Add a new directive "_When building the library, associated utilities are included by default. If these are not desired, then they can be disabled by adding -DBUILD_UTILS=OFF to the above cmake command._"
• Add a new directive "_When building the library, automated CI testing is included by default. If this is not desired, then it can be disabled by adding -DBUILD_TESTING=OFF to the above cmake command._"

[jbathegit]

Note that we may also need some new "12.1.0" entries in spack/package.py, but how do we get the corresponding sha256 hash code before actually creating the tag? In other words, is this a chicken vs. egg question?

[jbathegit]

Note that we may also need some new "12.1.0" entries in spack/package.py, but how do we get the corresponding sha256 hash code before actually creating the tag? In other words, is this a chicken vs. egg question?

Never mind, I found the answer to this myself in #552 and made the subsequent updates in #609