RFR: 8269700: source level for IntelliJ JDK project is set incorrectly

[Sergei Ustimenko]

After generating an Intellij project with the `idea.sh` script, IDEA might get confused 
with the language level settings in generated .idea/misc.xml. This results in a problem 
with modules discussed in https://mail.openjdk.java.net/pipermail/ide-support-dev/2021-June/000082.html.
To mitigate the problem changing the project's language level helps.

This patch sets language level for jdk and langtools projects to [X (Experimental Features)](https://github.com/JetBrains/intellij-community/blob/master/jps/model-api/src/org/jetbrains/jps/model/java/LanguageLevel.java) to avoid such problems.

As a side effect following term needs to be accepted in IDEA once opening the project for the first time:

You must accept the terms of legal notice of the beta Java specification 
to enable support for "X - Experimental features".  The implementation 
of an early-draft specification developed under the Java Community Process (JCP) 
is made available for testing and evaluation purposes only and is not 
compatible with any specification of the JCP.

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Commit messages:
 - 8269700: source level for IntelliJ JDK project is set incorrectly

Changes: https://git.openjdk.java.net/jdk/pull/4634/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4634&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269700
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4634.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4634/head:pull/4634

PR: https://git.openjdk.java.net/jdk/pull/4634