Add frozen string literal support for Ruby 3.4 compatibility

[moznion]

Issue # (if available)

n/a

Description of changes

I encountered the following warning message in my application with Ruby 3.4.7:

/opt/bundle-cache/ruby/3.4.0/gems/symmetric-encryption-4.6.0/lib/symmetric_encryption/header.rb:11: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

The reason for this warning is that Ruby 3.4 has begun to issue warnings for string mutations.

ref:

String literals in files without a frozen_string_literal comment now emit a deprecation warning when they are mutated. These warnings can be enabled with -W:deprecated or by setting Warning[:deprecated] = true. To disable this change, you can run Ruby with the --disable-frozen-string-literal command line argument.

https://www.ruby-lang.org/en/news/2024/12/25/ruby-3-4-0-released/

This pull request introduces support for frozen string literals as follows:

• Add # frozen_string_literal: true to all Ruby files
• Fix string mutations to ensure compatibility with frozen strings

This change improves memory efficiency and performance by making all string literals immutable by default, aligning with modern Ruby standards.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.