rename excludes dir

[pjfanning]

These MIMA excludes are associated with 1.1.x release changes

[raboof]

Are you sure this change is needed?

From https://github.com/apache/pekko/blob/main/CONTRIBUTING.md#binary-compatibility:

"If the violation can be ignored add exclude statements from the mima output to a new file named <module>/src/main/mima-filters/<last-version>.backwards.excludes/<pr-or-issue>-<issue-number>-<description>.excludes"

In other words it looks like the excludes directory should be named after the newest version that is incompatible with the current version. In this case 1.0.x seems correct then, as we expect to release 1.1.0 with this incompatible change, so we expect all 1.0.x versions to be incompatible with 1.1.0 and later.

[pjfanning]

I wouldn't be surprised if we have some excludes that do not follow the standard described in #180 (review) but I'll close this.