Renew Arcmutate OSS license (#1935)

And introduce an "Acknowledgements" section to the README, to thank
Arcmutate.

Author: Stephan202

README.md

@@ -272,6 +272,12 @@ guidelines][contributing].
 If you want to report a security vulnerability, please do so through a private
 channel; please see our [security policy][security] for details.
 
+## Acknowledgements
+
+This project's pull requests are analyzed using [Pitest][pitest], thanks to an
+OSS license generously provided by [Arcmutate][arcmutate]. Check them out!
+
+[arcmutate]: https://www.arcmutate.com
 [baeldung-java-source-target-options]: https://www.baeldung.com/java-source-target-options
 [bug-checks]: https://github.com/PicnicSupermarket/error-prone-support/blob/master/error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/
 [bug-checks-identity-conversion]: https://github.com/PicnicSupermarket/error-prone-support/blob/master/error-prone-contrib/src/main/java/tech/picnic/errorprone/bugpatterns/IdentityConversion.java

arcmutate-licence.txt

@@ -1,7 +1,7 @@
 # Arcmutate license for Error Prone Support, requested by sending an email to
 # support@arcmutate.com.
-expires=27/10/2025
+expires=20/10/2027
 keyVersion=1
-signature=aQLVt0J//7nXDAlJuBe9ru7Dq6oApcLh17rxsgHoJqBkUCd2zvrtRxkcPm5PmF1I8gBDT9PHb+kTf6Kcfz+D1fT0wRrZv38ip56521AQPD6zjRSqak9O2Gisjo+QTPMD7oS009aSWKzQ1SMdtuZ+KIRvw4Gl+frtYzexqnGWEUQ\=
 packages=tech.picnic.errorprone.*
+signature=JpOWM9DlGM2svuEdzHtHACu+Kjuinn7cMQ4J0tgKQ0dLgeC0BoYVRfBn/2KiH0E/kVfs3GznxOabgrOgjkxCSGOrYGsvd8zWZ2MaDbwC42/Njvmqth34j6gipyxhlnfKOdvHZ1VFZfoUeROY1ZzFfutR9G3vn30Dy7RSHFtK9kk\=
 type=OSSS