Update symfony/mime requirement from 8.0.* to 8.1.*

[dependabot]

Updates the requirements on symfony/mime to permit the latest version.

Release notes

Sourced from symfony/mime's releases.

v8.1.0

Changelog (symfony/mime@v8.1.0-RC1...v8.1.0)

• no significant changes

Changelog

Sourced from symfony/mime's changelog.

CHANGELOG

8.0

• Replace __sleep/wakeup() by __(un)serialize() on AbstractPart implementations

7.4

• Deprecate implementing __sleep/wakeup() on AbstractPart implementations; use __(un)serialize() instead

7.0

• Remove Email::attachPart(), use Email::addPart() instead
• Argument $body is now required (at least null) in Message::setBody()
• Require explicit argument when calling Message::setBody()

6.3

• Support detection of related parts if Content-Id is used instead of the name
• Add TextPart::getDisposition()

6.2

• Add File
• Deprecate Email::attachPart(), use addPart() instead
• Deprecate calling Message::setBody() without arguments

6.1

• Add DataPart::getFilename() and DataPart::getContentType()

6.0

• Remove Address::fromString(), use Address::create() instead
• Remove Serializable interface from RawMessage

5.2.0

• Add support for DKIM
• Deprecated Address::fromString(), use Address::create() instead

... (truncated)

Commits

• b164ae7 Add call to backers in README files
• 8d4e039 Merge branch '8.0' into 8.1
• 7c078c4 Merge branch '7.4' into 8.0
• a845722 Fix tests and merge resolution after merging 6.4 into 7.4
• 25b5570 Merge branch '6.4' into 7.4
• 7186d94 [String][Mime] Reject objects in typed-string properties during __unserialize
• 5575d37 [Routing][RateLimiter][Mime][Security] Harden __unserialize against __toStrin...
• 162a483 Merge branch '8.0' into 8.1
• 7d9a72b Merge branch '7.4' into 8.0
• b198dd6 Merge branch '6.4' into 7.4
• Additional commits viewable in compare view

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[wtorsi]

Superseded by the Symfony 8.1 release-line migration: main now pins this requirement at 8.1.* (merged via the 8.1 release PR).

[dependabot]

OK, I won't notify you again about this release, but will get in touch when a new version is available. If you'd rather skip all updates until the next major or minor version, let me know by commenting @dependabot ignore this major version or @dependabot ignore this minor version. You can also ignore all major, minor, or patch releases for a dependency by adding an ignore condition with the desired update_types to your config file.

If you change your mind, just re-open this PR and I'll resolve any conflicts on it.