Support frozen strings with REXML

[frederikspang]

Tests are passing with export RUBYOPT=--enable-frozen-string-literal

[johnnyshields]

@frederikspang I am pretty sure this is fixed on v2 already, but @pitbulk it might make sense to fix this on master anyway as it will is needed for Ruby 3.4

[frederikspang]

@johnnyshields I've adjusted some stuff from review. UUID_PREFIX is used as a constant, and .replace with Utils.set_prefix, which is not working out great :). I've replaced it with a variable. Should we deprecate or remove the (now unused) constant?

[johnnyshields]

@frederikspang yikes wow we were calling replace on a constant 🤦‍♂️ . The way you have it now with @@prefix is fine.

[pitbulk]

Changes are not compatible with ruby 2.X versions.

@johnnyshields, for this, the v2 should be used instead. we should address the UUID_PREFIX on that brach.

[johnnyshields]

@pitbulk frozen string stuff is handled already on 2.x. This is a "backport" in case we want it on 1.x branch, but the alternative here would be to just close this PR and tell users they must use 2.x for Ruby 3.4+. Which is reasonable in my opinion.

[johnnyshields]

Mutable constant addressed here: #735